For the fries. Preheat the oven to 220°C/425°F.
Place the sweet potatoes in a large bowl filled with ice water, let sit for 5 minutes. Drain and lay them between two clean kitchen towels. Massage a little, they should be as dry as possible. Place in a dry bowl, toss with thyme, cayenne pepper, garlic powder, smoked paprika and potato starch, season with salt. Spread the fries in an even layer on a baking sheet. Do not overcrowd the pan, if needed, divide the fries between 2 baking sheets. Transfer to the oven and bake for 15-20 minutes, flip and bake for 10-15 minutes more. You want the sweet potatoes to be tender, yet crisp.

In the meantime. Combine all the ingredients for the sauce in a small bowl.

Combine beef with heavy cream, onion powder, salt and pepper. Divide into 4 equal pieces. Using both hands, work meat into smooth balls, compacting very lightly. Press down to flatten each ball into a 1 inch/2 cm thick patty.

Heat a large, heavy-bottomed skillet over high heat. Melt the butter, add the patties and thinly sliced onion. Cook, without any touching, until edges are browned and you can see juices pooling in the center of patties, about 3 minutes. Then flip them over and place a slice of cheese on each burger. Cook for 2 minutes more. Never cook the burgers all the way through. The best doneness, is medium. The burger should have deeply caramelized crust on both sides, but you’ll still see juice oozing out from the mid-section. The skillet should be very hot to achieve this. Transfer burgers to a plate. Now place the buns into the same skillet, toast the buns, side down, until golden and crisp around the edges.

Start to build your burger. Spread a tablespoon of the sauce on bottom halves of the buns. Place a slice of tomato, lettuce, burgers and caramelized onions. Top with a tablespoon of sauce, close with the top bun, and serve with sweet potato fries and some extra sauce. Enjoy!