Preheat oven to 350°. Lightly coat the bottom and sides of a casserole dish or roasting pan large enough to fit all 10 squash with olive oil or cooking spray. Thinly slice points off bottoms of squash so they sit flat and level in pan. Flip the squash over and slice off the tops, reserving the tops to use as caps while cooking.
Gently scoop out and reserve inside flesh of each squash using a spoon or melon baller, being careful not to scoop all the way through the bottom. Roughly chop scooped squash and combine with onion, garlic and minced sage in a bowl.
Break sausage into pieces and brown in a pan over medium heat. Drain any excess grease and add stuffing mixture to pan]. Sauté for 5 minutes to soften onions. Incorporate cooked barley into stuffing mixture and sprinkle with salt and pepper to taste.
Generously stuff each pattypan squash with stuffing mixture, top each squash with the original cap and drizzle each top lightly with olive oil. Cover the entire pan with foil and roast in oven for 25 minutes. Remove foil and roast for another 10 minutes. Remove from oven and allow to cool for 5 minutes before plating. Garnish each stuffed squash with a whole sage leaf.