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Export 10 ingredients for grocery delivery
Heat oven to 375°F. Spray 3-quart baking dish with cooking spray.
In 12-inch skillet, heat butter over medium-high heat. Add onions and salt; cook 15 to 20 minutes, stirring frequently, until onions are dark golden brown. Add balsamic vinegar; cook and stir 1 to 2 minutes or until onions are dark brown.
In 5-quart Dutch oven, add whipping cream, thyme, sage, garlic and pepper. Heat to boiling over medium heat. Remove from heat; let stand 10 minutes. Remove thyme, sage and garlic. Return heat to medium; add sweet potatoes and Yukon Gold potatoes. Cook and stir about 5 minutes or until potatoes are hot.
Spread one-third of the onions in baking dish. Layer half of the potato mixture, another one-third of the onions and 1/3 cup of the Parmesan cheese. Repeat with remaining potato mixture and onions. Drizzle any remaining cream over onions, and sprinkle remaining 1/3 cup Parmesan cheese over top. Cover with foil; bake 15 minutes. Remove foil; bake 25 to 30 minutes or until potatoes are tender and browned and bubbly. Remove from oven; let stand 15 minutes before serving.