"Many people don't know that the crock inside a slow cooker is ovenproof and you can transfer a dish like lasagna into the oven (up to 400°F) for 10 minutes to brown the top," says Golino. Just be sure the pot is hot, to avoid drastic temperature changes that can cause it to crack.
1. Place onion, basil, oregano, and 2 whole garlic cloves in food processor; pulse 6 times, or until onion and garlic are chopped. Add tempeh, and pulse until tempeh is ground. Add tomatoes, oil, salt, and pepper; pulse until chunky sauce forms.
2. Combine spinach and minced garlic in bowl, and season with salt and pepper.
3. Coat bottom and sides of 4-quart slow cooker with oil. Spread 1 cup tomato sauce over bottom, and cover with layer of lasagna noodles, breaking them in pieces to fit around edges. Spread 3/4 cup spinach mixture over top. Place 1 layer roasted red peppers over spinach, and top with 3/4 cup ricotta. Spread 1 cup tomato sauce over ricotta. Sprinkle with 1/3 cup Swiss cheese. Repeat, layering noodles, spinach, peppers, ricotta, Swiss cheese, and tomato sauce. Top with 1 more layer noodles and remaining spinach, tomato sauce, ricotta, and Swiss cheese.
4. Cook 4 hours on high or 6 hours on low. Let stand uncovered 10 minutes before serving.