Roast the vegetables: Preheat the oven to 425°F (220°C). Place the sliced bell pepper, zucchini, and onion on a parchment-lined baking sheet. Drizzle with the olive oil and sprinkle with the turkey seasoning. Toss until evenly coated. Roast for 20–25 minutes, stirring halfway through, until tender and lightly caramelized.
Grill the turkey sausages: Preheat a barbecue or indoor grill to 400°F (205°C). Grill the turkey sausages for 12–15 minutes, turning occasionally, until nicely browned with grill marks and the internal temperature reaches 165°F (74°C).
Prepare the spread: Whisk together the mayonnaise, Dijon mustard, lemon juice, and garlic powder until smooth. Set aside.
Assemble the sandwiches: Spread the creamy Dijon mixture inside each bun, if using. Add one grilled turkey sausage to each bun and divide the roasted vegetables among the sandwiches.
Melt the cheese: Top each sandwich with a slice of provolone cheese. Place the sandwiches on a baking sheet and bake at 400°F (200°C) for 3–5 minutes, or until the cheese is melted and the buns are lightly toasted. Serve while still warm.