When modifying a recipe, i.e. halving a recipe, make sure to halve every ingredient! For Smitten Kitchen's Whole Lemon Tart, I used 1 egg instead of 1 egg + 1 egg yolk and 3/4 tablespoons cornstarch instead of 1.5 tablespoons of cornstarch. BUT, I used 1 stick of (unsalted, melted and cooled) butter instead of the half portion of 1/2 stick! The result: a very oily tart.
I did not realize my mistake until the tart had baked. I used paper towels to absorb as much butter-oil as possible. Given my mistake, the tart was tasty. Also, I added slices of mandarin - a sweet touch and the additional citrus helped to "cut" the butter-oil. I recommend this recipe but beware the butter.