Shoyu Peanut Tofu

package of extra-firm tofu (16 ounces), cubed
1/2 cup crunchy peanut butter
4 tablespoons soy sauce 
3 tablespoons lemon juice 
1/2 teaspoons ground ginger
2-3 cloves of chopped garlic
1/4 teaspoon crushed red pepper flakes
1 tsp sesame oil
handful of baby spinach (4 ounces)
8 oz of mushrooms
1 onion, chopped

Optional: 1 tbsp cornstarch or minute tapioca for thickening sauce

Combine all ingredients in slow cooker and cook for 2-4 hours on high. Add cornstarch to 1 tbsp cold water, then add to slow cooker, or add minute tapioca. Cook for another 10 minutes.