It’s terrible when you don’t get custody of the recipes. Sadly, we are unaware of any classic recipe for chicken thighs, carrots, onions, mustard powder, and soup in a casserole.
The fact is, there are millions of recipes in existence and a zillion ways to combine the ingredients (and presumably others) you mention. The soup reference makes us think that quite possibly yours is one of the many thousands of recipes the Campbell Soup Company has created to feature their products. We spent a little time exploring its recipe search engine, but it doesn’t allow you to search for specific ingredients or combinations of ingredients.
And without knowing which soup is used, we didn’t have time to wade through the 40 main dish recipes that include chicken and Cream of Chicken Soup or the 52 that use chicken and Cream of Mushroom Soup, etc. to come up with ones that might match your ex-girlfriend’s dish. The Campbell’s folks have recently made it more difficult to communicate with their Kitchen staff directly, but would be happy for you to pose your question in their online forums, in hopes someone there knows the dish you’re looking for.
Finally, we don’t mean to be catty, but maybe if you’d spent more time in the kitchen with her, you’d still have the recipe and the girlfriend