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.

Toss potatoes on a baking sheet with a large drizzle of oil, half the Ranch Spice (all for 4 servings), salt, and pepper. Roast on top rack until browned and tender, 18-20 minutes.

Meanwhile, heat a large drizzle of oil in a large pan over medium-high heat. Add mushrooms; 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 (if needed); season with half the thyme (all for 4 servings) and a pinch of salt and pepper. Cook, stirring, until veggies are browned and tender, 5-7 minutes.

Turn off heat; transfer to a plate and cover to keep warm. Wipe out pan.

Heat a drizzle of olive oil in same pan over medium heat. Add stock concentrates, half the garlic powder, and ¼ cup water (⅓ cup for 4 servings); cook, stirring occasionally, until slightly reduced, 2-3 minutes.

Halve baguettes lengthwise and place baguette halves directly on oven rack. Toast until golden brown.

In a small bowl, combine mayonnaise and remaining garlic powder. Season aioli with salt and pepper to taste.

Spread a thin layer of aioli over cut sides of toasted baguette halves (save any remaining aioli for serving). Using tongs or a slotted spoon, fill baguette halves with mushroom mixture; top with arugula. Close sandwiches; cut in half if desired.

Divide au jus between two small serving bowls (four small bowls for 4 servings).

Divide sandwiches and potatoes between plates. Serve with au jus and any remaining aioli on the side for dipping.