Adjust rack to top position and preheat oven to 425 degrees. Wash and dry produce.

Cut potatoes into ½-inch-thick wedges. Trim and thinly slice mushrooms (skip if your mushrooms are pre-sliced!). Halve, peel, and thinly slice onion.

In a small bowl, combine mayonnaise with horseradish sauce; season with salt and pepper.

Toss potatoes on a baking sheet with ¾ tsp garlic powder (1½ tsp for 4 servings), a drizzle of oil, salt, and pepper (reserve remaining garlic powder for another use).

Roast on top rack until browned and tender, 20-25 minutes.

Meanwhile, heat a large drizzle of oil in a large pan over medium-high heat. Add mushrooms and season with salt and pepper. Cook, stirring, until lightly browned, 4-5 minutes. TIP: Lower heat and add a splash of water if mushrooms begin to brown too quickly.

Add onion and another drizzle of oil; season with salt and pepper. Cook, stirring, until veggies are browned and tender, 5-7 minutes.

Stir in stock concentrate and 2 TBSP water; cook until water has evaporated and mixture is jammy.

Turn off heat; remove from pan and set aside. Wash out pan.

Spread half the sourdough slices with mustard, then top with even layers of Swiss (tearing to fit if necessary), mushrooms and onion, and Monterey Jack.

Top with remaining sourdough slices to create sandwiches.

Melt 1 TBSP butter in pan used for mushrooms and onion over medium heat. (For 4 servings, work in batches or use a second pan, using 1 TBSP butter for each batch.) Add sandwiches and push around in pan until melted butter has absorbed. Cook until bread is golden brown and cheese is slightly melted, 4-6 minutes.

Add another 1 TBSP butter to pan, then flip sandwiches and push around again until melted butter has absorbed. Cook until bread is golden brown and cheese melts, 4-6 minutes. TIP: If you have a heavy-bottomed pan, place on top of the sandwiches as they cook for a real panini experience!

Halve panini on a diagonal and divide between plates. Serve with potato wedges and horseradish aioli on the side for dipping.