Heat a large skillet over medium-high heat and add 1 tablespoon butter until melted.
Add the chicken chunks, season with the 1/4 teaspoon salt, and cook, stirring frequently, until cooked through about 5 minutes; transfer chicken to a plate and set aside.
Add a second tablespoon of butter to the skillet; add the sliced red onion and zucchini/squash and sauté, stirring often, until the vegetables are tender, about 3–4 minutes.
Add the halved grape tomatoes and the cooked chicken back to the skillet.
Add the remaining 2 tablespoons butter to the skillet and cook, stirring, until everything is heated through and combined, about 1–2 minutes.
Remove from heat and garnish with chopped parsley or basil if using, then serve.