Prepare pasta according to package instructions. Drain and allow to cool.
While pasta is cooking, dice onion, tomatoes, bell pepper, and mushrooms; cut pepperoni slices into quarters and chop spinach. Toss these ingredients into a large mixing bowl.
Add pasta to the mixing bowl with the veggies.
Mince garlic and add to a small bowl or dressing shaker, along with the remaining dressing ingredients; stir or shake until emulsified.
Pour dressing over the salad and toss to combine.
Serve at room temperature for best flavor; serve grapes on the side.