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 half the thyme leaves from stems (all the leaves for 4); mince leaves.

Heat a drizzle of oil in a large, preferably ovenproof, pan over medium-high heat. Add beef*; season with salt and pepper. Cook, breaking up meat into pieces, until browned and cooked through, 4-6 minutes. Turn off heat; transfer to a paper-towel-lined plate. Carefully discard any excess grease from pan. Wipe out pan. 

Heat a 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 to pan, then stir in carrots, diced onion, and salt. Cook, stirring, until veggies are softened, 5-7 minutes more.

Use pan used for beef here. 

Stir 1 TBSP butter (2 TBSP for 4 servings) into same pan, then add thyme, garlic powder, and flour. Cook, stirring, 1 minute. Stir in tomato paste until incorporated.

Add ¾ cup water (1 cup for 4) and stock concentrates, scraping up browned bits. Bring to a boil; reduce to a low simmer and cook until thickened, 2-3 minutes. Season with salt and pepper. TIP: If pan isn’t ovenproof, transfer mixture to a baking dish now.

After adding stock concentrates, return beef to pan; stir to combine. Cook through the rest of this step as instructed, adding splashes of water if filling seems too thick.

Mash drained potatoes with sour cream, half the cheddar (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 in an even layer, leaving a 1-inch border around edge of pan.

Sprinkle mashed potatoes with remaining cheddar.

Broil shepherd’s pie 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.