Beef and Onion Gratin with Mashed Potatoes -

This Beef and Onion Gratin with Mashed Potatoes is pure comfort: savory ground beef and sweet onions in a rich gravy, finished with a fluffy mashed potato topping that bakes up golden around the edges.

Why you’ll love this recipe

All-in-one, cozy casserole vibes without being complicated.
Family-friendly flavors: beefy, oniony, creamy, and lightly cheesy.
Great make-ahead dinner that reheats well for lunches.

Ingredients

2 lb Yukon Gold or russet potatoes, peeled and cut into chunks
6 tbsp unsalted butter, divided
1/2 to 3/4 cup milk (or half-and-half), warmed
1 1/2 tsp kosher salt, plus more to taste
Black pepper
2 tbsp olive oil (optional, if beef is very lean)
1 1/2 lb ground beef (85/15 works great)
2 large yellow onions, thinly sliced
3 cloves garlic, minced
2 tbsp all-purpose flour
1 1/2 cups beef broth
1 tbsp Worcestershire sauce
1 tsp dried thyme (or 1 tbsp fresh)
1 tbsp tomato paste
1 cup shredded sharp cheddar (optional but recommended)
Chopped parsley (optional, for finishing)

How to make Beef and Onion Gratin with Mashed Potatoes

You’ll build a simple beef-and-onion filling on the stove, then top it with mashed potatoes and bake until the top is lightly crisp and the edges bubble.

Step-by-step instructions

Prep and heat: Preheat oven to 400°F. Lightly butter or oil a 9×13-inch baking dish.
Boil potatoes: Add potatoes to a pot, cover with cold water, and salt generously. Bring to a boil and cook 12–15 minutes, until very tender.
Start the filling: While potatoes cook, heat a large skillet over medium-high. Add beef (and a little olive oil if needed). Cook until browned, breaking it up, 6–8 minutes. Season with salt and pepper. Spoon off excess fat if there’s a lot.
Cook onions: Add 2 tbsp butter and the sliced onions. Cook 8–10 minutes, stirring often, until softened and lightly golden. Add garlic and cook 30 seconds.
Thicken: Sprinkle flour over the beef and onion mixture and cook 1 minute, stirring to coat.
Make the gravy: Stir in broth, Worcestershire, thyme, and tomato paste. Simmer 3–5 minutes until glossy and thick enough to lightly coat a spoon. Taste and adjust salt and pepper.
Assemble: Spread the beef mixture in the baking dish. Stir cheddar into the filling if using.
Mash: Drain potatoes well. Mash with remaining butter and warmed milk until smooth and fluffy. Season to taste with salt and pepper.

Top: Spoon mashed potatoes over the filling and spread to the edges to seal. Use a spoon to make swirls on top (they brown nicely).
Bake: Bake 20–25 minutes until bubbling and lightly browned. For extra color, broil 1–2 minutes, watching closely.
Rest and serve: Let stand 10 minutes before scooping. Finish with parsley if you like.

Tips for the best gratin

Seal the edges: Spreading potatoes to the edges helps keep the filling from drying out.
For deeper onion flavor: Cook onions a few minutes longer until caramel-leaning, then proceed.
Control thickness: If the filling seems thin, simmer a couple extra minutes. If it’s too thick, splash in a bit more broth.
Cheese options: Sharp cheddar is classic; Gruyère or a cheddar-jack blend also works.

Make-ahead and storage

Make ahead: Assemble fully, cool, cover, and refrigerate up to 24 hours. Bake at 400°F until hot throughout, 30–40 minutes (cover with foil if browning too fast).
Refrigerate: Store leftovers covered up to 4 days.
Freeze: Freeze tightly wrapped up to 2 months. Thaw overnight in the fridge before baking to reheat.