Heat your oven to 400°F (205°C). Line a baking sheet with parchment for easy cleanup.
Slice a shallow rectangle down the center of each whole-wheat roll. Scoop out some of the interior, leaving a 1/2-inch wall on all sides. Reserve the crumbs for meatballs, croutons, or compost if you're virtuous.
Lightly brush the insides with olive oil and a pinch of salt. Bake 5–7 minutes until the edges start to crisp. This prevents sogginess. You're building a foundation, not a raft.
In a skillet over medium heat, add remaining olive oil. Sauté onion and bell pepper 3–4 minutes until softened. Add garlic and smoked paprika; cook 30 seconds. Toss in spinach until just wilted. Season to taste.
Quickly sear turkey sausages or hot dogs in the same pan until lightly browned. Browning = flavor. Slice in half lengthwise if you prefer more surface area.
Stir mustard (and ketchup or marinara if using) into the veggie mix to unify the filling. This helps the cheese bond and adds punch.
Sprinkle a little cheese in each toasted boat. Add the veggie mixture, nestle the sausage on top (or tuck slices in), then finish with more cheese.
Return to the oven for 6–10 minutes, until cheese is melted and bubbly and the bread is crisp at the edges.
Shower with chopped parsley or chives. Hit with a crack of black pepper. Serve hot with a side salad or carrot sticks if you're doing the whole “balance” thing.