Chicken Ratatouille
1 (6 ounce) can tomato paste 1 cup chicken stock 1 green zucchini, cut in half lengthwise and … 1 red bell pepper, sliced in 1/4-inch strips … 1 tablespoon herbes de Provence 1 yellow squash, cut in half lengthwise and i… 2 tablespoons olive oil, or as needed 2 teaspoons celery salt 2 teaspoons dried oregano 2 teaspoons ground cumin 3 (10 ounce) chicken breasts, cubed, or more … 3 cloves garlic, or more to taste, minced 6 brown mushrooms, sliced and halved, or to t… salt and ground black pepper to taste ¼ cup thinly sliced red onion