Melt butter with olive oil in a large nonstick skillet over medium-high heat; add shallots, and sauté 2 minutes. Add zucchini; sauté 5 minutes or until zucchini is tender. Stir in minced garlic, and cook 1 minute. Remove from heat; stir in lemon juice, lemon zest, kosher salt, and freshly ground black pepper. Toss in cooked pasta, fresh mint, walnuts, and Parmesan cheese. Sprinkle with feta cheese just before serving.