1. Preheat the oven to 375°F (190°C).

2. Peel the onions and chop them finely then sauté in olive oil until they are translucent.

3. Cook the garlic cloves in a separate pan until fragrant then add the milk stirring occasionally until the mixture thickens.

4. Combine the cooked spinach ricotta cheese and a pinch of salt and pepper in a large bowl and mix until the ingredients are fully incorporated.

5. Spread a thin layer of tomato sauce at the bottom of a 9x13 inch (23x33 cm) baking dish followed by a layer of lasagna sheets.

6. Spread half the spinach and ricotta mixture followed by half the grated cheese and half the sautéed onions.

7. Repeat the layers starting with the tomato sauce then the lasagna sheets the remaining spinach and ricotta mixture the remaining grated cheese and finally the remaining sautéed onions.

8. Cover the dish with aluminum foil and bake for 30 minutes.

9. Remove the foil and bake for an additional 15 minutes or until the top layer is golden brown.

10. Remove from the oven and let it rest for 10 minutes before serving.