Season chicken with salt and pepper. Heat oil in deep skillet over medium-high heat. Add chicken thighs and cook, not turning them until well-browned, about 4 minutes. Flip and brown other side for 3 minutes. Transfer to plate.
Add onion and mushrooms, 1/2 teaspoon salt, and 1/4 teaspoon pepper, and cook over high heat, stirring occasionally, until vegetables begin to brown, about 6 minutes. Stir in tomatoes. Add chicken to pan and bring to a simmer. Cover and simmer over medium heat 8 to 10 minutes, or until chicken is cooked through.
Use a combination of portobello and button mushrooms, if you prefer.