• Melt butter and extra virgin olive oil in a large skillet over medium heat. Add onions, season lightly with salt, then saute until starting to become tender, 10 minutes. Add dried thyme then continue to saute until onions are very tender and slightly caramelized, 15-20 minutes, stirring occasionally. Add garlic and sage then saute until garlic is fragrant, 2-3 minutes.
• Add milk to the skillet, season generously with salt and pepper, then turn heat up to bring mixture to a bubble. Turn heat back down to medium then simmer until milk is just slightly thickened, 2-3 minutes (don't over-reduce.) Set aside.
• Preheat oven to 350 degrees then spray a 9x13" baking dish with nonstick spray. Layer half the sweet potatoes in tight rows in the bottom then season with salt and pepper. Using a slotted spoon, strain the onions from the milk then scoop onto potatoes and spread into an even layer. Top with remaining potatoes, season with salt and pepper, then drizzle reserved sauce on top (there won't be much.) Cover dish with foil then bake for 20 minutes. Remove foil then bake for 15 more minutes. Potatoes should be crisp tender when inserting a knife into them - if they're still hard, bake for another 5 minutes.

For the bread crumb topping:

• Rinse then dry skillet used for sauce then place over medium-heat. Melt butter then add sage, bread crumbs, and a little salt. Saute until lightly toasted them add to a bowl and, once slightly cooled, stir in parmesan cheese (if using.)
• Sprinkle bread crumb mixture on top of sweet potatoes then bake for an additional 10-15 minutes or until bread crumbs are evenly toasted and sauce is bubbly. Let dish sit for 10 minutes then serve.