Add the shallot to the pan and cook, stirring occasionally, until soft and fragrant, about 2 minutes. Add the garlic and cook, stirring, until fragrant, 1 minute more. Stir in the stock and simmer until the liquid is reduced by half, 4-5 minutes. Reduce heat to low, then stir in 4 tablespoons of the butter, capers, lemon zest, lemon juice and the remaining 1/2 teaspoon of salt.