Make Sauce. Once the oil is heated and shimmering, add ¼ cup of diced yellow onion and sauté until fragrant and tender, or about 4 to 5 minutes. Then, pour in 21.5 ounces of condensed cream of chicken soup, 10 ounces of RoTel diced tomatoes with green chiles, and 1 ½ tablespoons of taco seasoning. Stir to combine all the sauce ingredients.