Preheat the oven to 400 degrees.
Prep all of the vegetables.
Toss tomatoes, peppers, onion, and garlic with olive oil, salt, and pepper and arrange on a baking tray lined with baking paper.
Place the chorizo sausage on top of the vegetables. Bake for 40-45 minutes, or until chorizo skin is crisp and the vegetables are soft and cooked through.
Place the diced potatoes in a pot of salted cold water and bring to a boil over medium-high heat. Cook until the potatoes are fork-tender, about 6-8 minutes. Drain well.
Place the potatoes in a single layer on a plate or baking sheet to allow them to cool more rapidly. Refrigerate to speed up the cooling process.
Once the potatoes are cooled, mix in the onion, celery, carrot, scallions, and mayonnaise; season with salt & pepper to taste, then serve.
Split rolls lengthwise by cutting through the top and heat in the oven for two minutes. Pile roast vegetables into each bun and top each with chorizo.
Drizzle with mustard and serve with potato salad and sparkling water.