Preheat oven to 400 degrees.
Heat a large skillet over medium heat. Add the olive oil to the skillet.
Dump all of the vegetables into the skillet and cook, stirring often, for 5 minutes. Season with salt.
Remove pan from heat and set aside.
Add the butter to a small saucepan over medium heat and let melt.
Once melted, whisk in the flour and cook 1 minute, whisking constantly.
Slowly whisk in the vegetable broth and continue whisking until mixture has thickened to the consistency of a gravy.
Remove from heat and whisk in sour cream.
Pour sauce over vegetables and stir to combine. Pour mixture into 8x8 baking dish.
Top with shredded cheddar and sweet potato puffs.
Bake for 30 minutes. Serve immediately.