Related to How would you use a lot of lemons?, but at the opposite scale:
I have a recipe that needs 1 Tbsp. lemon zest, so I've zested several areas of skin off this lemon and I'm not sure what I can usefully do with the rest.
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I typically juice it and put it in a drink. You can make a simple Lemon Drop Martini with lemon juice, vodka and simple syrup. Whenever I bake a Dominican cake, I always have leftover lemons (and lemon zest).
Cool down the simple syrup to about room temperature, mix all three ingredients and stir until mixed.
To serve, make a paste with kosher salt, simple syrup and lemon juice, then line the rim of the serving glass (a martini/cocktail glass) with the paste. Pour in the drink either plain or on the rocks / over ice.
If you have leftover lemon zest, you can make lemon pepper, which goes great with chicken or fish. To make homemade lemon pepper, put equal parts of lemon zest and ground black pepper, mix and bake flat for a few minutes. I tend to do:
Mix in a small bowl, then lay flat on a baking sheet and bake at 350 (preheated) for 10 to 15 minutes. When done, let it air cool and save in an airtight container. To use, rub it into chicken or fish.