On a heavy-duty cutting board, use a meat mallet/tenderizer hammer to hammer the chx. breasts into thin breasts, about 3/8" each. This will tenderize them, and they will absorb the flavor of the orange and rosemary better, once you break down the tissues. Trim off all fat and sinew. Set aside while you prepare the orange and warm your pan.
NOTE: You DON'T want to hammer these on your glass platter. LOL! My photo shows them on a platter AFTER I have tenderized them on a cutting board. Put the chx. breasts into a polyethylene bag (the kind cereal box liners are made of), so the chx. doesn't "spatter" all over your counter, etc. I save these cereal box liners just for this purpose. Waxed paper, saran wrap, WalMart bags, etc., are all too flimsy and break easily when you're hammering on the chicken, and you'll end up w/ a spattered mess on your backsplash, countertops, etc. :o/
Remove zest, and extract juice from the orange. You'll want at least a few tablespoons of the orange juice, perhaps even a 1/4 to 1/3 of a cup, and all of the zest. Set both aside. If you need to use two oranges, go for it! This is one of those "eyeball it" things, or "to your desired taste" issues. I like it "orange-y", so I use a lot of the orange juice.
Put chx. breasts into a large bowl, and sprinkle with orange juice from your fresh orange(s). Toss to coat. Let it marinate for several minutes in juice.
Lay chx. breasts out on a large flat plate or surface. Salt and pepper, to taste. Let them sit while you prepare the pan.
Heat a large skillet to MedHigh; add 1 tsp. olive oil, and 1 tsp butter, and allow to melt together. (The olive oil helps raise the burning temp. of the butter - slightly! Be watchful that the butter doesn't turn "brown").
Preheat your oven to 400°.
Brown chx. breasts, only two at a time in the pan, about 3 minutes per side. When both sides are golden, transfer to a square baking dish and keep warm. They don't need to be fully cooked in the browning pan; they will finish cooking in the oven.
Repeat process with the remaining 2 chx. breasts & remainder of olive oil and butter. Remove to the baking dish and keep warm, when done.
Lower the heat to Med or MedLow. Into the pan, add your fresh squeezed/pressed garlic, and saute' for about a minute, or until glossy. DO NOT OVER COOK...garlic burns very easily!
Deglaze the pan with the remainder of the fresh orange juice, chicken stock, and balsamic vinegar, gently loosening any stuff that stuck to the bottom of the pan.
Pour dezlazing juices over all of the chicken breasts.
Sprinkle the orange zest (about a TBSP., more or less to your taste) over all of the chicken breasts, spreading evenly.
Tuck pieces of the fresh rosemary sprigs here and there, between the chicken breasts, and sprinkle a few sprigs on top, if desired.
Bake at 400° for about 15 - 20 minutes, until juices run clear when pricked w/ a fork.
Arrange on a platter. Pour juices from pan over all the chicken on the platter, to keep chicken moist. Garnish with fresh orange slices and rosemary, if desired.