I make this differently in winter than summer. In summer, I usually make my own sauce, but in winter, it's semi-homemade or from jars. Also, I don't always use meat. I think lasagna is best without it. But when I do use meat, I use a little extra sauce.
Yesterday I decided to try it in the slow cooker. I cooked it on high for a little less than 3 hours, then broiled the top in the oven. I am not sure the amount of time that would be best if you cook it on low, but if you have a timer, you can set it to turn on and then off 30 minutes before serving.
Here's how I did it yesterday in a 7 quart (about 7.5 liter) slow cooker, to serve 6 with leftovers:
1/2 large onion, medium dice
1 1/2 lbs mild Italian sausage, casing removed
Cooked the onion in a large pan with olive oil (I buy large bottles of extra-virgin at Costco and use it in just about everything,) then added sausage, broken up and stirred til almost done.
2 jars of Wegman's pasta sauce (they had it on sale for 89 cents recently so I bought lots. It has only nice ingredients in it,) stirred in. Plus most of another jar for assembly, which I would not use if I left out the sausage.
This part is mostly not negotiable. It is the martini recipe of lasagna filling; simple, classic, always correct.
4 cups mozzarella cheese, grated + another 2/3-1 cup
3 cups whole milk ricotta cheese
1/2 cup asiago cheese, grated (usually I use parmiggiano, and romano would be okay.)
2-3 tbs mixed Italian seasoning (adjust depending on the sauce you're using)
Also: 12 pieces lasagna pasta, uncooked. (unless using oven, then cook in boiling water for only about 5 minutes.) In a lasagna pan you need 16.
Put a little sauce in the bottom of the pan, add 3 pieces lasagna, broken to fit. Obviously you don't need to break them if you're using a standard lasagna pan (or two cake pans, with one less layer in each.) Layer sauce, then 1/3 cheese mixture over it, rinse, repeat twice. Add the final 3 pieces of lasagna, spoon over some sauce, and then enough mozzarella (and more of the other cheese if you like) to lightly cover the top.
If you bake it in the oven, you bake it at 350º covered for about 50 minutes, then remove the foil to brown it for another 15-20 minutes. Let it cool before serving.
For the slow cooker, I added a little extra sauce around the perimeter to make sure the noodle edges would be cooked. I could tell it was done and ready to be browned by how it looked all melded together, and stuck a knife through the middle to check the noodles. It went through easily at just under 3 hours, but I could tell it was not mushy. Then I put the crock in the oven and browned it on the low broiler setting for about 5 minutes.
If your slow cooker is smaller than 6 quarts, this recipe will be too large. I would suggest comparing this recipe to another casserole one you like to adjust quantities. If you make this recipe using 9x13 cake pans, you can freeze one for later. You can cook it for the first 45 minutes, let it cool for a few minutes, then seal it well to freeze, thaw and reheat later. Or you can freeze it uncooked, as well—it'll take a couple of hours to cook from frozen that way.
Leftovers for lunch? Are the best. Even though this picture really is not.