Heat the oven to 400 degrees Fahrenheit. Melt the butter in a heavy pan and cook the shallots gently until soft. Stir in the paprika and cook for 2-3 minutes. Add the tomatoes, season with salt, and cook until the mixture thickens. Stir in the sour cream.

Spread a layer of potatoes in a gratin dish and cover with some of the tomato mixture; repeat the layers. Pour the stock over. Cover the dish and bake for 1 hour.

Uncover, return the dish to the oven, and continue to bake until the potatoes are tender and the liquid is almost all absorbed.