Take 2 handfuls of spinach and cook it in the hot pan with some olive oil for 40 seconds. Then, put your cooked spinach into the baking tray.
Chop two onions and 2 stems of leeks. Place them straight into the pan. Then, add some ground pepper and 2 cloves of sliced garlic. After that, add 5 sliced mushrooms. Add some salt and stir for 15 minutes.
In 15 minutes the stuffing is done. Let it cool down.
Next, make simple white sauce. In the same pan where you made mushrooms, add 3 tablespoons of olive oil and 3 tablespoons of plain flour. Give that a good mix. Slowly add 800 ml of milk, continuously mixing the sauce. When the sauce is homogeneous, let it cook on low heat.
While the sauce is cooking, fill your cannelloni. Then, place them into the baking tray.
Add 100 g of grated cheddar into the white sauce. Then, pour your sauce over the cannelloni. Put some slices of mushrooms on the top and a little drizzle of olive oil for decoration.
Cook in the oven at 180 degrees C for 45 minutes until golden.