Make the sauce first. Drain the tomatoes (reserve the juice) and put them into a food processor; chop to a rough puree. (You can also do this with your fingers in a bowl.)
Heat the olive oil in a saucepan over medium heat till the oil shimmers. Saute the garlic and crushed red pepper for about a minute--until you can smell the garlic.
Add the tomatoes, 1/2 cup of the reserved juice, basil, oregano, and salt. Bring to a boil, and lower the heat to medium-low. Simmer for about half an hour to as long as an hour (longer cooking will thicken the sauce, to your taste), uncovered, stirring now and then.
Remove the Italian sausage from casing and blend with ground round and chopped onions in a skillet over medium heat. Add 1/4 cup water. Cook while breaking the meat up with a kitchen fork to avoid clumping. Drain to remove excess fat and set aside.
If using dried noodles, bring a large pot of water to the boil with 1 Tbs. salt and 1 Tbs. olive oil. Add the lasagna noodles and boil for about two minutes, or until the noodles are no longer stiff. Remove to a bowl of cold water for a moment, then remove and drain. (If using fresh pasta, omit this step.)
Cook the spinach in the same water you used to cook the pasta, just for a minute. Remove with a slotted spoon, drain, and spread out.
In a glass or ceramic baking dish (about 9"x13"x4"), pour about 1/4 cup of olive oil on the bottom and spread it up the sides. Make the following layers:
Cover the casserole with foil. Put the casserole in the center of the oven and bake for two hours. Insert a meat thermometer or fork into the center and check to see that the inside is very hot. If not, leave it in the oven another half hour.
Remove the foil and bake another 15 minutes or so, until the top gets slightly crusty.
After removing from the oven, let the lasagna rest for at least 15 minutes before attempting to slice. Use a sharp knife to slice, then remove pieces starting from the center with a metal spatula. Spoon two tablespoons of sauce over the lasagna on its serving plate.