To poach mahi: Place lemon and onion slices and peppercorns in the bottom of a large skillet. Place mahi on top and add enough water to cover lemon and onion slices. Bring to a boil; cover and simmer for about 8-10 minutes or until fish flakes with a fork. Cut mahi into bite-sized pieces and set aside.
To cook vegetables & gnocchi: In a large skillet, heat oil over med-high. Add squash and garlic and season with salt and pepper. Cook, stirring occasionally, until squash is crisp-tender, 4-5 minutes. Add tomatoes and cook, stirring occasionally, until juicy, about 2 minutes.
Meanwhile, in a large pot of boiling salted water, cook gnocchi according to package directions. Reserving 1/2 cup of cooking liquid, drain gnocchi and transfer to skillet. Toss gnocchi with vegetables, adding enough cooking liquid to create a sauce. Remove from heat and stir in basil, cheese, butter and lemon juice. Add mahi and serve.