Bring a large pot of lightly salted water to a boil. Cook fusilli pasta in the boiling water, stirring occasionally until cooked through but firm to the bite, 12 minutes. Drain and rinse under cold water.

Whisk olive oil, vinegar, dill, garlic, salt, and black pepper together in a bowl.

Combine cooked pasta, mushrooms, arugula, Gruyere cheese, and onion together in a large bowl. Pour dressing over pasta mixture; toss to coat. Cover bowl with plastic wrap and refrigerate pasta salad at least 2 hours before serving.