In the sauté pan, melt the butter and add the canola oil. Sauté the onions and garlic over medium heat until the onions are soft and glistening. Add the tomato paste, paprika, and crushed red pepper. Stir and add the sliced mushrooms. Cook until the mushrooms have softened slightly, about 3 to 4 minutes.
Line the bottom of the slow cooker with the chicken thighs, layering as necessary. Sprinkle on some kosher salt and a few grinds of fresh black pepper, and then the marjoram. Transfer the cooked onion and mushrooms to the slow cooker, covering the chicken with it. Add the chicken stock and put the lid on top.
Select the Auto setting on your slow cooker and cook for about 6 hours. If your slow cooker does not have an auto setting, you can cook on High for 2 hours then Low for 4 hours, or you can cook on Low for 7 to 8 hours. The chicken will be very tender and possibly falling off the bone.
Remove the chicken pieces, place them on a warm plate, and cover. In the small bowl, whisk together the sour cream and cornstarch. Add a couple tablespoons of the sauce to the sour cream mixture and stir. In the slow cooker, whisk in a few tablespoons of the sour cream mixture at a time until the sour cream is dissolved. Put the chicken pieces back in and cover with the lid. Cook on high for about 30 minutes more.
Place chicken pieces in a large bowl and pour the sauce over it. Garnish with snipped chives and serve.