Heat olive oil in a skillet over medium-high heat.
Sear the chicken thighs for 3-4 minutes on each side until golden brown.
Transfer the seared chicken into the Crockpot.
Add the diced bell pepper, onion, garlic, and sliced mushrooms to the Crockpot.
Pour in the crushed tomatoes and tomato paste. Stir to combine all the ingredients.
Season the mixture with oregano, basil, rosemary, thyme, salt, and pepper.
Optional: Add olives and capers for extra flavor.
Cover and cook on low for 6-8 hours or on high for 3-4 hours.
Once the chicken is fully cooked and tender, serve with your choice of keto-friendly sides like zoodles or cauliflower rice.