Crumble beef into a 2-quart microwave-safe casserole pan and stir in the chopped onion. Cover loosely and microwave on high for 4-6 minutes until the beef reaches 155°F. Stir every 2 minutes.
Once the meat is cooked, stir to break up the meat and drain the extra fat into a spare bowl and throw away when cool.
Stir in the spinach, cheese, yogurt, salt and pepper. Cover and set aside.
Warm tortillas in the microwave if desired.
Spoon 1/3 cup filling into the center of each tortilla and fold up. Lay flat in the casserole pan with the seam facing down.
Mix all ingredients for the enchilada sauce in a microwave-safe container. Cover and Microwave on high for 4-6 minutes, stirring every couple of minutes.
Pour sauce over tortillas, cover, and microwave again on high for 2-4 minutes.
Garnish with shredded cheese.