Heat the vegetable oil in a skillet over medium heat. Stir in the onions, and cook until they begin to soften, about 3 minutes. Increase the heat to medium-high, and add the sliced sirloin. Cook and stir until the meat is no longer pink, and beginning to brown around the edges, about 7 minutes.
Add the potatoes, cream of mushroom soup, and milk. Bring to a simmer, then reduce the heat to medium-low, and simmer 10 minutes. Season to taste with salt and pepper before serving.