Preheat the oven to 400F and line a large sheet pan with parchment paper (or two smaller sheet pans). The food needs enough room for roasting without steaming, so make sure there's plenty of room for the veggies and sausage to spread out (no packed layers).
If the fingerling potatoes are small, use them as-is. If larger, halve the potatoes into 1" pieces. If you're using yellow baking potatoes, halve and then cut into smaller pieces. The goal is for all the veggies to be relatively uniform in size so everything cooks evenly.
1 1/2 pounds fingerling potatoes
Cut the ends off the peppers and pull out any seeds and membranes. Halve the peppers.
Add the oil, garlic cloves, and all seasonings to the vegetables. Mix well with your hands. (If you don't have the individual spices on hand, alternatively, use 2 teaspoons Italian Seasoning.)