diff --git a/changelog.d/20230926_172753_codewithemad_config_loaded_action.md b/changelog.d/20230926_172753_codewithemad_config_loaded_action.md
new file mode 100644
index 0000000..588d4da
--- /dev/null
+++ b/changelog.d/20230926_172753_codewithemad_config_loaded_action.md
@@ -0,0 +1 @@
+- 💥[Feature] New action introduced: CONFIG_LOADED. This actions is called whenever the config is loaded. (by @CodeWithEmad)
\ No newline at end of file
diff --git a/tutor/commands/config.py b/tutor/commands/config.py
index abcdc03..58e1833 100644
--- a/tutor/commands/config.py
+++ b/tutor/commands/config.py
@@ -114,7 +114,7 @@ class ConfigListKeyValParamType(ConfigKeyValParamType):
     type=ConfigListKeyValParamType(),
     multiple=True,
     metavar="KEY=VAL",
-    help="Append an item to a configuration value of type list. The value will only be added it it is not already present. (can be used multiple times)",
+    help="Append an item to a configuration value of type list. The value will only be added if it is not already present. (can be used multiple times)",
 )
 @click.option(
     "-A",
@@ -147,16 +147,18 @@ def save(
     env_only: bool,
 ) -> None:
     config = tutor_config.load_minimal(context.root)
-    config_full = tutor_config.load_full(context.root)
     if interactive:
         interactive_config.ask_questions(config)
     if set_vars:
         for key, value in set_vars:
             config[key] = env.render_unknown(config, value)
     if append_vars:
+        config_defaults = tutor_config.load_defaults()
         for key, value in append_vars:
             if key not in config:
-                config[key] = config_full.get(key, [])
+                config[key] = config[key] = config.get(
+                    key, config_defaults.get(key, [])
+                )
             values = config[key]
             if not isinstance(values, list):
                 raise exceptions.TutorError(
diff --git a/tutor/config.py b/tutor/config.py
index a5e1eca..d343394 100644
--- a/tutor/config.py
+++ b/tutor/config.py
@@ -1,4 +1,5 @@
 from __future__ import annotations
+from copy import deepcopy
 
 import os
 
@@ -26,6 +27,15 @@ def load(root: str) -> Config:
     return load_full(root)
 
 
+def load_defaults() -> Config:
+    """
+    Load default configuration.
+    """
+    config: Config = {}
+    update_with_defaults(config)
+    return config
+
+
 def load_minimal(root: str) -> Config:
     """
     Load a minimal configuration composed of the user and the base config.
@@ -51,6 +61,7 @@ def load_full(root: str) -> Config:
     update_with_base(config)
     update_with_defaults(config)
     render_full(config)
+    hooks.Actions.CONFIG_LOADED.do(deepcopy(config))
     return config
 
 
@@ -319,3 +330,19 @@ def _update_enabled_plugins_on_unload(_plugin: str, _root: str, config: Config)
     Note that this action must be performed after the plugin has been unloaded, hence the low priority.
     """
     save_enabled_plugins(config)
+
+
+@hooks.Actions.CONFIG_LOADED.add()
+def _check_preview_lms_host(config: Config) -> None:
+    """
+    This will check if the PREVIEW_LMS_HOST is a subdomain of LMS_HOST.
+    if not, prints a warning to notify the user.
+    """
+
+    lms_host = get_typed(config, "LMS_HOST", str, "")
+    preview_lms_host = get_typed(config, "PREVIEW_LMS_HOST", str, "")
+    if not preview_lms_host.endswith("." + lms_host):
+        fmt.echo_alert(
+            f'Warning: PREVIEW_LMS_HOST="{preview_lms_host}" is not a subdomain of LMS_HOST="{lms_host}". '
+            "This configuration is not typically recommended and may lead to unexpected behavior."
+        )
diff --git a/tutor/hooks/catalog.py b/tutor/hooks/catalog.py
index 2f62810..66ed97a 100644
--- a/tutor/hooks/catalog.py
+++ b/tutor/hooks/catalog.py
@@ -57,6 +57,13 @@ class Actions:
     #: :parameter str name: docker-compose project name.
     COMPOSE_PROJECT_STARTED: Action[[str, Config, str]] = Action()
 
+    #: This action is called at the end of the tutor.config.load_full function.
+    #: Modifying this object will not trigger changes in the configuration.
+    #: For all purposes, it should be considered read-only.
+    #:
+    #: :parameter dict config: project configuration.
+    CONFIG_LOADED: Action[[Config]] = Action()
+
     #: Called whenever the core project is ready to run. This action is called as soon
     #: as possible. This is the right time to discover plugins, for instance. In
     #: particular, we auto-discover the following plugins: