My neighbor served this for dinner, and everyone couldn’t stop talking about it

Add the onions, potatoes and garlic, season with salt and pepper, and bring to a simmer.

Cook until potatoes are tender.

3. Drain the potatoes and onions through a colander set over a bowl to reserve the cream. Remove and discard the garlic clove.

4. Transfer the potatoes to a greased 9-by-13-inch casserole dish.

Sprinkle with half of the cheese, and drizzle with 1 cup of the reserved cream. Discard the rest.

5. Combine the olive oil and mustard in a bowl.