Trim tops and ends off onions. Peel outer layer, then slice into 1/8in rounds.
Melt 1/4 cup butter in 12" cast iron skillet over medium heat.
Once butter is melted and bubbling, place onions into the skillet and stir to coat the them in butter.
Lower heat to medium-low. Sprinkle onions with about 1 tsp kosher salt or to taste.
Let onions sit and cook for about 5 minutes without stirring.
Stir onions and cook for an additional 40 minutes, stirring every 5-10 minutes. While the onions are cooking, prepare the garlic and peaches.
Once onions are softened and browned, add chopped garlic and sliced peaches. Stir to combine and let cook for about 5 more minutes.
Top with a sprinkling of finishing salt and freshly ground pepper.
Serve directly from the skillet. Or use for pizza topping or burger topping, along with goat cheese.