Heat olive oil in a skillet over medium-high heat. Add chicken and onion; saute until chicken is still slightly pink in the center, 3 to 5 minutes. Add yellow squash, green zucchini, red bell pepper, and mushrooms. Cook and stir until slightly softened, about 4 minutes. Add garlic; cook, stirring frequently, until fragrant, about 3 minutes more.
Pour chicken stock into skillet and add tomato paste. Stir in herbes de Provence, cumin, oregano, and celery salt. Stir well. Continue to cook and stir, adding more liquid if necessary, until vegetables are cooked to your preference, 4 to 8 minutes more. Season with salt and pepper.
