In a large skillet, saute your chicken breasts - adding onion & garlic if desired. Cook until the chicken is done & nicely browned. Set aside.
In a large pan, melt butter over medium-high heat. Whisk in flour; cook and stir for 2 minutes. Add broth, salsa, & evaporated milk. Continue to cook & stir until thick and bubbly. Stir in the chicken, reduce heat to low.
Meanwhile, in a bowl, combine biscuit mix, milk, & ¾ cup cheese.
Pour salsa verde chicken mixture into a 9x13 dish. Sprinkle with ¼ cup of cheddar cheese if desired. Top with large spoonfuls of biscuit dough.
Bake, uncovered, at 400 degrees for 20 minutes, or until biscuits are golden and chicken mixture is hot and bubbly.