Adjust racks to top and middle positions and preheat oven to 425 degrees. Wash and dry all produce. Cut potatoes into 1/2-inch-thick wedges; toss on a baking sheet with a drizzle of olive oil, half the Italian Seasoning (you’ll use the rest later), salt, and pepper. Roast on top rack, tossing halfway through, until golden brown, 20-25 minutes.

Place sausage on a second lightly oiled baking sheet. Roast on middle rack, flipping halfway through, until browned and cooked through, 14-16 minutes. Transfer to a cutting board. Once cool enough to handle, thinly slice on a diagonal.

Meanwhile, halve, peel, and slice onion. Core, deseed, and thinly slice bell pepper. Slice baguettes lengthwise, stopping before you get all the way through (they should look like hot dog buns when you’re done). Place 2 TBSP butter (4 TBSP for 4 servings) in a small microwave-safe bowl; microwave until softened, 10-20 seconds. Stir in garlic powder, salt, and pepper.

Heat a drizzle of oil in a large pan over medium-high heat. Add onion and bell pepper. Season with salt, pepper, and remaining Italian Seasoning. Cook until softened and lightly browned, 6-8 minutes. Stir in sliced sausage, stock concentrate, mustard, and 1/4 cup water (1/3 cup for 4 servings). Bring to a simmer and cook until slightly thickened, 1-2 minutes. Turn off heat.

Meanwhile, spread cut sides of baguettes with garlic butter. Place on baking sheet used for sausage. Toast in oven until lightly golden, 3-5 minutes (you’ll be toasting again once you assemble the subs). Let cool slightly.

Carefully fill each toasted baguette with sausage mixture. (TIP: If you’re left with extra filling, serve it on the side!) Top with mozzarella. Bake on middle rack until cheese is melted, 3-5 minutes. Divide between plates with potato wedges. Serve with ketchup on the side for dipping.