Heat a medium-sized skillet over medium-high heat and add butter. Add onion and mix. Add brown sugar and mix. Stir onions occasionally until butter is absorbed and onions are browned, 20 to 30 minutes.

Preheat the oven to 450 degrees F (230 degrees C).

Meanwhile, place kale in a large skillet over medium heat. Stir every couple of minutes until kale is wilted and dark green, about 10 minutes.

Place pizza crust on a baking sheet. Spread caramelized onions evenly over crust. Spread kale evenly over crust. Spread tomato slices over pizza. Sprinkle feta cheese on top.

Bake in the preheated oven for 8 minutes, checking to make sure kale does not burn.