Butter an 8-cup baking dish.

Preheat oven to 350°F (175°C).

Chop onions finely.

Saute chopped onion in 2 tbsp butter until translucent.

Thinly slice kale.

Blanch kale in a large pot of boiling salted water until tender.

Quench blanched kale in ice water.

Transfer kale to a bowl using a slotted spoon.

Return water to a boil and add coarsely chopped potatoes.

Cook potatoes until tender.

Drain potatoes and return them to the pot.

Add remaining butter (excluding the amount used for sauteing onions), minced garlic, and sour cream to the potatoes.

Mash potatoes until smooth.

Chop leek.

Mix leeks, half of the sauteed onion, and blanched kale into the mashed potatoes.

Add half of the grated cheddar cheese and mash into the potato mixture.

Season the mixture to taste with salt and pepper.

Spoon the rumblethumps mixture into the prepared baking dish.

Sprinkle the remaining cheddar cheese evenly over the top.

Place the remaining sauteed onion in a 2-3 inch wide strip down the center of the dish.

Bake for about 35 minutes, or until the cheese is bubbly and golden brown.