Halve peppers lengthwise, remove cores, rinse and salt. Rinse the spinach, cut off the stems, and place in a large pot (while still wet). Heat the spinach over low heat just until wilted, then remove and pat dry. Peel and chop the onion and garlic, fry in the butter, add the spinach and sautée for a few minutes. Stir in the whipping cream.
Grate the parmesan, mix half of it with the spinach and season with lemon juice, salt and pepper. Fill the pepper halves with this mixture and place on a greased baking dish. Sprinkle remaining cheese on the spinach and bake in a preheated oven at 175°F (approximately 350°F) for 10 minutes. Heat tomato sauce, served with the stuffed peppers and garnish with basil.