Get Ready: Preheat your oven to 400°F (204°C). Grab a medium baking sheet and line it with parchment paper for easy cleanup.
Roast the Cauliflower: In a big bowl, gently toss the cauliflower florets with 2 tablespoons of olive oil, 1/4 teaspoon of sea salt, and 1/4 teaspoon of black pepper. Spread them out evenly on your prepared baking sheet. Pop them into the hot oven and roast for about 25-30 minutes, giving them a flip halfway through, until they're beautifully golden and tender. This step is key for flavor!
Start the Meat Sauce: While the cauliflower roasts, heat the remaining 1 tablespoon of olive oil in a large skillet over medium heat. Add the chopped onion and cook for 5-8 minutes until it softens and starts to turn golden.
Brown the Beef: Add the ground beef to the skillet. Season it with the remaining 1 teaspoon of sea salt and 1/4 teaspoon of black pepper. Cook for 8-10 minutes, breaking it up with a spatula until it's nicely browned. Drain any excess fat for a healthier dish.
Simmer the Sauce: Stir in the minced garlic, marinara sauce, and 1/4 cup of fresh basil. Let it simmer gently for 2 minutes to let all those delicious flavors meld together, then take it off the heat.
Prepare the Cheesy Layer: In a separate medium bowl, mix the ricotta cheese, grated Parmesan cheese, and the optional egg until everything is smooth and creamy. The egg helps keep it together.
Assemble Your Lasagna: Once the cauliflower is ready, transfer it to a 9x9 inch (23x23 cm) baking dish, spreading it evenly across the bottom. Spoon the savory meat sauce over the cauliflower. Next, carefully dollop spoonfuls of the ricotta mixture over the meat sauce (about 9 dollops works well) and gently spread them out. Finish with a generous sprinkle of shredded mozzarella cheese.
Bake to Perfection: Bake your cauliflower lasagna for 10-15 minutes, or until it's hot and bubbly. If you love a golden-brown top, you can briefly place it under the broiler for 1-2 minutes at the very end. Before serving, sprinkle with a little more fresh basil for extra freshness!