Adjust racks to top and middle positions and preheat oven to 425 degrees. Wash and dry produce.

Cut potatoes into ½-inch-thick wedges. Halve and peel onion; very thinly slice one-quarter (one-half for 4 servings). Finely chop remaining onion. Quarter lime. Roughly chop pulled mushrooms into bite-size pieces.

Toss potatoes on a baking sheet with a large drizzle of oil, 1 tsp Southwest Spice Blend (2 tsp for 4 servings), salt, and pepper. Roast on top rack until browned and tender, 20-25 minutes.

Meanwhile, in a small microwave-safe bowl, combine sliced onion, juice from half the lime, ¼ tsp sugar (½ tsp for 4 servings), and a pinch of salt. Tightly cover with plastic wrap. Microwave until onion is softened, 1 minute. Set aside.

In a separate small bowl, combine mayonnaise with a squeeze of lime juice. Season with salt; stir to combine.

In a medium bowl, combine BBQ sauce, ketchup, stock concentrate, half the cornstarch, remaining Southwest Spice Blend, and ¼ cup water (all the cornstarch and ½ cup water for 4 servings). Stir until smooth. 

Heat a large drizzle of oil in a large pan over medium-high heat. Add chopped onion and pulled mushrooms; season with salt and pepper. Cook, stirring, until veggies begin to soften, 6-7 minutes (as mushrooms soften, break up larger pieces using your cooking spoon). TIP: If veggies begin to brown too quickly, reduce heat to medium.

Add ¼ cup water (½ cup for 4 servings) to pan with onion and mushrooms; stir in BBQ sauce mixture. Cook, stirring constantly, until sauce has thickened, 30-60 seconds. TIP: If sauce seems too thick, stir in another splash of water.

Taste and season with salt and pepper. Turn off heat.

Place buns directly on middle rack and toast until warmed through, 2-3 minutes. 

Carefully halve toasted buns and divide between plates; spread cut sides of bottom buns with lime mayo. Fill buns with BBQ mushroom filling and as much pickled onion (draining first) as you like. Serve potato wedges and mixed greens alongside sliders; drizzle greens with dressing.