Preheat the oven to 400 degrees. Coat a baking sheet with cooking spray.
Roast the veggies by placing the tomatoes, mushrooms, onion chunks, garlic cloves (with skins on), and baby potatoes on the baking sheet. Drizzle with olive oil and season with sea salt and freshly cracked pepper.
Roast for 20-25 minutes, stirring occasionally.
Add the asparagus and roast for 10 minutes or until the asparagus and potatoes are fork-tender. Remove from the oven. Carefully remove & discard the garlic skins, slice, and toss with the roasted veg. Cover with a foil tent to keep warm.
Meanwhile, cook the sausage in a large skillet over medium heat for 10 minutes or until done.
Prepare the dressing by whisking together the vinegar, olive oil, mustard, water, and garlic, then season with sea salt and freshly cracked pepper to taste, until well combined.
Finish preparing the spinach salad by tossing the warm veggies and sliced sausages with the spinach and dressing until evenly coated. Top with feta cheese.