Add flour to onion mixture and make a light colored roux, about 2-3 minutes over medium high heat. Remove roux from heat and grate in one clove of garlic stirring into the roux, but still off the heat
Add a ladle or two of chicken stock from the kale and potato to the roux, stir to combine, then add the roux all at once to the kale and potato mixture and stir until combined with the stock and the vegetables.
Bring the mixture to a simmer and cook over medium heat until the sauce thickens, about 3-4 minutes.
Remove from heat and stir in 2 heaping tablespoons of sour cream. Keep warm over low heat until ready to serve, do not allow to simmer after adding the sour cream.