Cut the squash into bite-sized pieces. Heat the oil in a large skillet over medium heat. Working in batches, cook the squash, flipping until browned, about 5 minutes. Transfer to a bowl.
Whisk together the maple syrup, vinegar, mustard, rosemary, Worcestershire sauce, pepper flakes, 2 1/4 cups water, 1/2 teaspoon salt and a few grinds of pepper in a medium bowl.
Return the skillet to medium heat and pour in the maple mixture, scraping up any brown bits from the bottom of the pan with a wooden spoon. Bring to a boil and return the squash to the pan. Reduce the heat to maintain a low simmer and cook, stirring occasionally, until the liquid almost completely evaporates and the squash is cooked through and glazed, about 25 minutes.
Remove from the heat, add the butter and gently stir until melted. Season with salt and pepper. Adjust the consistency of the glaze with 1 to 2 tablespoons of hot water if too thick.
Copyright 2016 Television Food Network, G.P. All rights reserved.