The tricky thing about making lemon curd is carefully emulsifying the egg and lemon juice together. If you don't stir enough or heat the curd too fast, your eggs will break into tiny pieces of scrambled egg, adding a grainy texture to your curd. The secret is to go slow and stir often. You can make a double boiler by adding a metal pan over a saucepan or work at very low heat. I also recommend straining your curd through a fine-mesh sieve after you make it to remember any accidental clumps. If you get grainy curd, try blending it with an immersion blender, which may help break up the chunks. Once I switched to using instant clearjel in this lemon curd recipe, I haven't had any issues with grainy lemon curd because it doesn't have to cook for very long!