1. Peel and finely chop the onion. In a sauté pan, heat a little olive oil and sauté the onion for a few minutes.
2. Then add the lentils and paprika, season with salt and pepper, mix well and simmer for 10 minutes.
3. Meanwhile, prepare the béchamel sauce. Dissolve the cornstarch in a little rice milk, pour into a saucepan with the remaining rice milk. Place over medium heat and thicken the mixture while whisking. Remove from heat, season with salt and pepper.
4. Preheat the oven to 200°C (400°F).
5. In a gratin dish, pour a little tomato sauce into the bottom, place two lasagna sheets, then a layer of lentil mixture, followed by a layer of béchamel sauce. Repeat the layers, finishing with a third layer of lasagna sheets, a little tomato sauce, and béchamel sauce. Sprinkle with cheese of your choice. Bake for 30 to 35 minutes.