Preheat oven to 400°.
In a large cast-iron skillet or heavy-bottomed sauté pan, heat 1 tablespoon oil over medium-high heat. Season chicken breast halves with salt and pepper. Place chicken skin side down and cook for approximately 5 minutes, until skin is golden brown. Lower heat and turn chicken over. Add 1 tablespoon butter and baste chicken, cooking for an additional 2 minutes. Place chicken in baking dish and place in oven. Cook chicken until internal temperature reaches 165°.Wipe sauté pan.
Meanwhile, bring 4 cups water to boil in medium saucepot. Add diced sweet potatoes and lower heat to simmer. Cook until tender, approximately 7 minutes. Drain and set aside.
In sauté pan heat 2 tablespoons vegetable oil over medium heat. Add onions and cook 3–4 minutes, stirring occasionally. Add sweet potatoes, season with salt and pepper and cook until golden brown.
In separate medium sauté pan, heat Brussels sprouts and ½ cup chicken broth over high heat.When liquid has almost completely evaporated, add another ½ cup of chicken broth plus 1 tablespoon butter. Cook until liquid has evaporated again. Brussels sprouts should still have a bite to them. Remove from pan and set Brussels sprouts aside in a warm place.
Add remaining chicken broth to pan with remaining 1 tablespoon butter. Remove chicken from oven. Add any drippings from baking dish to the broth and set pan over high heat. Reduce liquid to 1⁄3 cup. Remove from heat. Add sage and stir.
To serve, arrange chicken on six plates with Brussels sprouts and top with sweet potato. Drizzle with sauce. Garnish with sage leaves.
Recipe by From Chef Dave Johnson, Local 121, Providence