Preheat the oven to 400°F (200°C) and line a baking sheet to roast eggplant and zucchini until tender and lightly golden, about 20 minutes.

Warm 2 tbsp olive oil in a skillet, add chopped onion and garlic, cook until fragrant and translucent, about 5 minutes.

Arrange sliced bell peppers on the baking sheet, toss with oil, roast until slightly charred around the edges, about 15 minutes.

Let roasted veggies cool briefly on a wire rack before layering.

In a medium saucepan, heat 1 tbsp olive oil over medium heat, add crushed tomatoes, oregano, garlic, salt, and pepper; stir well.

Simmer the sauce gently over medium-low heat until thickened and vibrant red, about 10 minutes.

In a saucepan over medium heat, melt butter; whisk in flour vigorously to form a smooth roux for 2 minutes.

Gradually pour in warm milk, whisk until silky and thick, about 5 minutes, then stir in nutmeg and Parmesan.

Spoon a thin layer of tomato sauce into mini dishes, layer with veggie slices, repeat layers, then top with béchamel.

Bake at 375°F (190°C) until béchamel is golden and bubbling, about 18–20 minutes.