Preheat the oven to 400°F/200°C.
Without peeling the garlic, slice the whole head of garlic in half and drizzle some olive oil on the exposed surface, sprinkle a pinch of salt, close the garlic and wrap it up with aluminum foil and place on a baking sheet. Roast in the oven for 45 minutes.
While the garlic is roasting, chop the bell peppers, zucchini, asparagus, and cremini mushrooms into 1-inch pieces. Toss with olive oil and lay them out on a baking sheet in an even layer.
Take the garlic out of the oven once it's finished roasting, and put the chopped vegetables in the oven and roast for 25-30 minutes. Toss halfway through.
Boil a large pot of water and add a good pinch of salt. Add the pasta and cook according to package directions, adding a minute to the cooking time.
While the pasta is cooking, add the roasted garlic, balsamic vinegar, extra virgin olive oil, and salt in a large bowl and whisk until combined. Add the cooked and drained pasta along with the chopped onion, olives, and roasted veggies to the bowl and toss. Add black pepper to taste, and let it cool slightly before adding the baby greens.
Let the pasta salad rest for 30 minutes to an hour before serving it, to let the flavors meld.