Place an oven rack directly under the broiler and turn on the oven to the broil setting.
Melt the ghee in a large oven-safe skillet over medium heat. Add the onions and cook for 5 to 7 minutes, until they become translucent and begin to brown. Add the garlic and cook for 1 to 2 minutes, until the garlic starts to turn golden brown. Remove the onions from the pan and set aside.
Season each side of the sole with the spice blend. Add the sole to the pan and cook for 1 to 2 minutes, until it starts to brown. Flip the sole and return the onions to the pan, placing them around and on top of the fish. Add the pasta sauce and capers to the skillet around the fish.
Transfer the pan to the oven and broil for 5 minutes, until the fish is white all the way through and flaky, or cooked to your liking. If desired, add a squeeze of fresh lemon juice before serving.