Place chicken between two sheets of plastic wrap.
With a meat mallet, pound chicken to 1/4-inch thickness.
Sprinkle chicken with salt and pepper.
Place 1 sage leaf on top of each chicken breast half.
Top each side of the chicken breast with two prosciutto slices.
Roll chicken and secure with kitchen twine at both ends.
Melt butter with oil in large nonstick saute pan over medium-high heat.
Sear chicken on both sides and finish cooking in the oven at 350 degrees for 12 to 15 minutes.
Keep fat in pan and remove chicken to a large platter, cover and keep warm.
Using the same pan (add more oil if necessary), cook onions until they start to brown.
Add broccoli florets, sugar snap peas, carrots and red bell peppers, and cook until still slightly crisp.
Deglaze pan with sherry vinegar.
Season to taste with salt and pepper.
Toss in fine herbs and mix thoroughly.
Spoon vegetable medley over chicken saltimbocca.