Dice the Yukon gold potatoes without peeling them. The skin is thin enough to leave on.

Place the potatoes in a pot of cold water and add 1/2 teaspoon salt. Bring to a boil over medium high heat, then reduce and cook until fork tender, about 7 to 9 minutes depending on the size of your pieces.

Heat a large skillet over medium heat. Add the browned ground beef and diced onion and cook 4 to 5 minutes until the onions soften. If starting with raw ground beef, brown it first with salt and pepper, then add the onions.

Rinse the frozen mixed vegetables under warm water to thaw, then add them to the skillet and cook about 5 minutes.

Add 2 tablespoons butter and the minced garlic to the skillet and let the butter melt. Sprinkle in the thyme.

Sprinkle 2 tablespoons flour over the mixture and cook 1 to 2 minutes, until the flour disappears into the butter.

Pour in the beef broth and cook about 2 minutes, until the sauce thickens slightly. Remove from heat and spread into an 8x8 baking dish.

Drain the potatoes and return them to the pot. Add 3 tablespoons butter and mash with a potato masher or fork.

Add the heavy cream, milk, and shredded cheese. Mix until combined, then season with salt and pepper to taste.

Spread the mashed potatoes over the meat mixture all the way to the edges, leaving peaks and swirls on top rather than smoothing it flat. Sprinkle with a little pepper.

Bake at 425°F for 25 to 30 minutes, until the potato peaks are golden and crispy. Place a sheet pan underneath the dish to catch any drips.

Let stand a few minutes before serving.