Slice the squash, zucchini, and carrot into julienne strips--about the size of kitchen matchsticks.
In a large skillet, heat the butter and saute the vegetables and the garlic for about a minute, until tender but still crisp. Remove and keep warm.
Mix the flour, salt and pepper. and dust the chicken breasts lightly with the flour, seasoned with the salt and pepper.
Add the olive oil to the skillet where you cooked the vegetables, and heat it until it shimmers. Sear the chicken breasts, turning once, until browned. Remove the chicken to a pan and put it in the preheated oven to cook the rest of the way. and keep warm.
Add the wine, lemon juice, mustard and Worcestershire to the pan. Bring to a light boil, and heat until bubbling. Add the prosciutto and reduce the sauce by about half.
Return the vegetables to the skillet and toss to coat with the sauce. Serve the chicken with the vegetables underneath.