Melt butter in a large skillet over medium heat. Add sliced onions and season with salt to release juices. Cook until onions are golden brown (approximately 25 minutes), stirring occasionally.
Add garlic, thyme (fresh or dried), and season with salt and pepper. Reduce heat to low-medium and add arrowroot starch (or flour) and quickly coat onion mixture with the flour.
Add half & half and whisk to combine everything, keeping heat at low-medium. Cook and whisk occasionally for approximately 5 minutes. Sauce should slightly thicken.
Preparing Butternut Squash
Peel butternut squash and cut lengthwise. Remove seeds and pith. Thinly slice crosswise creating half moons.
Assembling Butternut Squash Gratin
Spoon ⅓ of the onion cream mixture to the bottom of the casserole dish. Arrange butternut squash slices across the dish in several layers.
Pour remaining onion cream mixture, creating pockets between the pan sides and slices to allow the sauce to drip through.
Bake uncovered for 40 minutes on the middle rack. Broil for the last 5 minutes of cooking time to brown the tops. Remove from oven and allow to rest for 5 minutes for sauce to settle before serving.
Video
Notes
Two tablespoons of fresh thyme is approximately 3-4 sprigs of thyme. Alternatively, you can use 2 teaspoons of dried thyme.
Arrowroot flour is used to thicken the cream sauce. Tapioca can be used as well. If not gluten free, any all purpose flour can be used.
To catch possible overflow of the creamy sauce, set a baking sheet with lip on the bottom rack during cooking.
The information shown above is an estimate provided for your convenience by an online calculator. It should not be considered as a substitute for a professional nutritionist's advice. Please see our complete Nutritional Information Disclaimer.