Boil lasagna sheets until partially tender and drain.
Sauté chopped zucchini, bell pepper, garlic paste, oregano, and tomato sauce in olive oil for 8 minutes.
Melt butter, stir in flour, and gradually whisk in milk to prepare smooth white sauce.
Layer red vegetable sauce, lasagna sheets, white sauce, and mozzarella in a baking dish.
Repeat layers and bake at 190°C for 25 minutes until top is golden and bubbling.