Fill a large pot with water, cover, and set over high heat. When the water is boiling, add the lasagna sheets and cook until al dente, about 8 minutes. When the lasagna sheets are done cooking, drain and set aside. Preheat the oven to 350*F.

While the lasagna sheets are cooking, heat the olive oil over medium heat in a large saucepan until shimmering. Add the onions, bell peppers and 1 cup of spinach and stir to mix, cooking until the onions have started to cook through and soften, about 3 minutes. Add the tomato paste and stir to combine, then add the crushed tomatoes. Turn the heat to low and let the sauce cook and thicken while you prepare the rest of the lasagna, about 10 minutes. Stir the sauce often to keep from burning on the bottom of the pan.

While the sauce is cooking, combine the ricotta cheese, parmesan cheese, egg, and salt and pepper in a large bowl and whisk until the ricotta is smooth. Fold in the spinach.

Lay the lasagna sheets out side by side on a clean counter or sheet pan. Remove the sauce from the stovetop and add 1/2 cup of sauce to the bottom of a 7"x7" or 7"x5" ovenproof baking dish, spreading evenly. Spread 1-2 tablespoons of ricotta mixture onto each lasagna sheet and roll from bottom to top. As each sheet is rolled around the ricotta mixture, place it upright in the baking dish on top of the sauce. The curls of each roll should be facing upwards. Add the rolls, side by side and one at a time, until all the sheets have been stuffed and rolled.

Pour the remaining tomato sauce on top of the rolls, using a spoon to gently push some sauce down into each roll. Top the dish evenly with the shredded cheese.

Bake the lasagna rolls for 10-12 minutes, or until the cheese has melted and it is starting to bubble. Remove from the oven and serve hot.