In medium sauté pan, cook sausage until rendered and well browned. Remove and reserve.
Add onions and cook for 5 minutes, or until translucent.
Add butter until melted; then whisk in flour. Cook for 1 minute, stirring constantly. Add milk and chicken stock, whisking thoroughly. Add in reserved sausage. Cook over medium heat until thickened, about 5 minutes. Add more liquid if necessary to thin. Season to taste.
Preheat the oven to 375 degrees F (190 degrees C).
Heat vegetable oil in an over-safe skillet over high heat. Add smashed fingerling potatoes and cook until crispy and well browned, about 5 minutes per side.
Top potatoes with Sausage Gravy and cheese curds. Bake until warmed through and cheese is melted, about 15 minutes.