Heat olive oil and butter in a large frying pan.

Add onion and sauté for five minutes. Add garlic and sauté one minute more. Add mushrooms and continue sautéing five more minutes, stirring frequently. Raise the heat during the last two minutes so the liquid evaporates.

Transfer the sautéed vegetables to a large bowl. Add spinach, cottage cheese, shredded cheese, egg, and seasoning. Mix together well.

Transfer filling to a piping bag. Cut off the tip so it fits the opening of the cannelloni tubes. When filling the tubes, place the tip of the bag at one end of the cannelloni and use your finger to close off the other end.

Melt the butter in a frying pan.

Add flour and cream and whisk together. Stir until creamy and thickened. Remove from heat.

Add seasoning and adjust to taste.

Preheat oven to 350 degrees Fahrenheit (175 degrees Celsius).

Grease a nine x 13-inch (20×30-centimeter) baking pan with butter.

Pour half the marinara sauce on the bottom of the pan. Add half the béchamel sauce in dollops on top. Lay filled cannelloni one near the other in the sauce (you may need another small pan). Repeat layers of sauce, covering all the tubes.

Cover well and bake for 40 minutes.

Remove from oven and sprinkle with shredded cheese.

Cover and return to the oven for another five minutes.

Uncover and bake for five more minutes. Let sit for a few minutes, covered, before serving.