Chicken Ratatouille

Name: Chicken Ratatouille
Ingredients: 2 tablespoons olive oil, or as needed; 3 (10 ounce) chicken breasts, cubed, or more to taste; 0.25 cup thinly sliced red onion; 1 yellow squash, cut in half lengthwise and into 1/4-inch slices; 1 green zucchini, cut in half lengthwise and into 1/4-inch slices; 1 red bell pepper, sliced in 1/4-inch strips and halved; 6 brown mushrooms, sliced and halved, or to taste; 3 cloves garlic, or more to taste, minced; 1 cup chicken stock; 1 (6 ounce) can tomato paste; 1 tablespoon herbes de Provence; 2 teaspoons ground cumin; 2 teaspoons dried oregano; 2 teaspoons celery salt; salt and ground black pepper to taste