Heat the olive oil in a skillet over medium heat. Add the chopped onion and garlic, sautéing until the onion is translucent, about 5 minutes.
Place the chicken thighs in the crockpot. Season with salt, pepper, oregano, basil, and red pepper flakes if using.
Add the sautéed onion and garlic, sliced bell pepper, diced tomatoes, tomato paste, and chicken broth to the crockpot. Stir to combine.
Cover and cook on low for 6-8 hours or on high for 3-4 hours, until the chicken is tender and cooked through.
Once cooked, shred the chicken in the crockpot using two forks and stir to combine with the sauce.
Serve over cooked pasta or rice and garnish with fresh parsley.