For the ground beef filling, combine ground beef, onion, garlic, tomato paste, tomato, egg, chili flakes, oregano, basil, sage, pepper, and salt in a large mixing bowl until fulling incorporated. Set aside.
Remove the whole stem from the cauliflower making sure to leave the head fully intact.
Boil the cauliflower in a pot of water over high heat for 8-10 minutes or until fork-tender.
Remove the cauliflower from the pot and place on parchment paper-lined baking sheet.
Carefully remove any remaining stem making sure there is space for the filling.
Fill the head of cauliflower with ground beef mixture making sure to press the filling into the head of cauliflower.
Cover inverted head of cauliflower with aluminum foil and bake in preheated oven for 30 minutes, or until ground beef mixture is cooked through.
Preheat oven to broil.
For the bechamel, melt butter in a saucepan over medium heat.
Mix in flour and stir until mixture lightly browns, about 2 minutes.
Stir in the milk and allow mixture to thicken, about 5 minutes.
Add in the Parmesan, salt, and pepper and mix until fully incorporated and has the consistency of a thick batter.
Remove aluminum foil from stuffed cauliflower.
Cover the cauliflower in bechamel sauce and a sprinkle of Parmesan cheese.
Broil for 5 minutes or until bechamel lightly browns.