• Adjust rack to top position and preheat oven to 425 degrees. Wash and dry produce. • Cut potatoes into 1/2-inch-thick wedges. Trim and thinly slice mushrooms. Halve, peel, and thinly slice onion. • In a small bowl, combine mayonnaise with 1/4 tsp garlic powder (1/2 tsp for 4 servings); season with salt and pepper. (You’ll use the rest of the garlic powder in the next step.)

• Toss potatoes on a baking sheet with remaining garlic powder, a drizzle of oil, salt, and pepper. • Roast on top rack until browned and tender, 20-25 minutes. While potatoes roast, pat steak dry with paper towels and season all over with salt and pepper. Heat a drizzle of oil in a large pan over medium-high heat. Add steak and cook to desired doneness, 5-7 minutes per side. Turn off heat; transfer to a cutting board to rest for at least 5 minutes. Wipe out pan. Thinly slice steak against the grain.

• 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. Use pan used for steak here.

• 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. Fill sandwiches with steak.

• 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 the real panini experience! • Halve panini on a diagonal and divide between plates. Serve with potato wedges and garlic aioli on the side for dipping.