Place the onions in large pot over medium-high heat, with butter and a generous pinch of kosher salt. Cook until the butter melts into the onions and steam begins to come off. Reduce heat to low. To ensure and expedite softening, I cover the onions for about 35 minutes with a lid to build steam and keep the moisture in, stirring every 12-15 minutes. Once you remove the lid, cook for 20 minutes, stirring a few times. During the last 20 minutes, add the white wine and allow it to cook off. Continue cooking until the onions have reached your desired level of caramelization.
Make the Crostini:
While the onions caramelize, prepare the crostini. Use a pastry brush to lather olive oil onto both sides of the bread. Line the slices on a baking sheet and place on the center rack of the oven. Turn the broiler to 450. Toast slices for 2-4 minutes (depending on your broiler), then remove, flip the bread, and toast the other side for an additional 2-3 minutes.
Allow bread to cool, then spread on goat cheese, caramelized onions, and thyme leaves. Serve warm or chilled.
Notes
TIP: To quickly and evenly cut the onions, use a mandolin! I like this one.TIP: You can skip adding the wine, but just add more water as needed toward the end of cooking so the onions don’t dry out and burn. You can cook the onions until they are very dark brown (this obviously just takes more time) for deeper flavor.