Wash and dry produce.

Dice potatoes into ½-inch pieces; place in a large pot with enough salted water to cover by 2 inches. Bring to a boil and cook until tender, 15-20 minutes.

Reserve ½ cup potato cooking liquid, then drain and return potatoes to pot.

Keep covered off heat until ready to mash.

While potatoes cook, trim and quarter mushrooms. (Skip if your mushrooms are pre-sliced!) Trim, peel, and finely dice carrots. Halve, peel, and dice half the onion (whole onion for 4 servings). Strip thyme leaves from stems.

Heat a large drizzle of olive oil in a large, preferably ovenproof, pan over medium-high heat. Add mushrooms and a big pinch of salt. Cook, stirring occasionally, until lightly browned, 5 minutes.

Add another drizzle of olive oil, carrots, and onion; season with salt. Cook, stirring, until veggies are softened, 5-7 minutes more.

Stir 1 TBSP butter (2 TBSP for 4 servings) into pan with veggies until melted, then add garlic powder, flour, and half the thyme (all for 4). Cook, stirring, 1 minute.

Stir in tomato paste until incorporated, 1 minute.

Add ¾ cup water (1 cup for 4) and stock concentrates, scraping up any browned bits from bottom of pan. Bring to a boil, then reduce to a low simmer. Cook until thickened, 2-3 minutes. Season generously with salt and pepper. Turn off heat. TIP: If your pan isn’t ovenproof, transfer mixture now to a baking dish.

Mash drained potatoes with sour cream, half the Monterey Jack (you’ll use the rest in the next step), and 2 TBSP butter (3 TBSP for 4 servings) until smooth and creamy, adding splashes of reserved potato cooking liquid as needed. Season with salt and pepper.

Heat broiler to high.

Once filling has thickened, spoon mashed potatoes on top. Spread into an even layer, leaving a 1-inch border around edge of pan.

Sprinkle potatoes with remaining Monterey Jack.

Broil until cheese is lightly browned, 3-4 minutes. TIP: Watch carefully to avoid burning.

Let rest at least 5 minutes, then divide between plates and serve.