Heat the oil in a saucepan, add onion and garlic and saute for 1 minute
Add tomatoes and simmer for 5-10 minutes
Place the tomato mixture in a blender and blend until well combined, set aside
Melt the butter in a saucepan and add the flour. Whisk the flour into the melted butter until a thick paste forms. Gradually add 1 cup of milk and stir until sauce starts thickening. Add the last cup of milk and continue stirring until the sauce starts thickening again. Set aside
Start layering the lasagne in the following order. Half mushrooms, half feta, half silverbeet, half tomato sauce, half pasta, half white sauce. Repeat the process with the remaining ingredients. Sprinkle cheese over the top.
Place lasagne in oven and bake for 30min or until the pasta is cooked.