Bring a large pot of salted water to a boil and cook the rotini pasta according to package directions until al dente. Drain the pasta and set aside. I always reserve about 1/2 cup of pasta water before draining—it's helpful if the salad needs any loosening later.

In a small bowl, whisk together the balsamic vinegar, extra virgin olive oil, maple syrup, minced garlic, dried oregano, and red pepper flakes until well combined. Set the dressing aside at room temperature.

Heat 1.5 tbsp olive oil in a large pot or deep skillet over medium-high heat. Add the diced red onions and cook for about 5 minutes, stirring occasionally, until they begin to soften. Add the mushrooms and salt, cooking for another 3-4 minutes until the mushrooms release their moisture and start to brown. Then add the halved cherry tomatoes and cook for 2 minutes until they just begin to warm through.

Pour the balsamic vinaigrette from Step 2 into the pot with the cooked vegetables and stir well to combine. Add the sundried tomatoes and kalamata olives, then bring the mixture to a gentle simmer for 3-4 minutes, allowing the flavors to meld and the vegetables to absorb the dressing. I like to let this simmer just long enough to warm everything through without overcooking the delicate vegetables.

Add the cooked pasta from Step 1 and the baby spinach to the pot with the warm vegetable mixture, stirring gently to combine. Cover the pot and let it sit for 2-3 minutes to allow the residual heat to wilt the spinach while keeping the pasta and vegetables warm. Stir once more to ensure everything is evenly coated with the dressing.

Transfer the warm pasta salad to a serving bowl or individual plates. Taste and adjust seasoning if needed. The salad can be served warm as-is, or allow it to cool to room temperature for a refreshing cold pasta salad. If it seems too thick after cooling, stir in a splash of the reserved pasta water to loosen the dressing.