How do I log this?

I would make the whole thing, weigh it on the food scale, and then create the "servings" as one-ounce by weight portions. So if it weighs 18oz, I'd call the whole recipe 18 servings and then weigh my portions so if I ate 2.5oz, I'd log it as 2.5 servings. Does that make sense?

^^^^^ that's how I do it!

Like others said, MFP online has a "Recipe" tab. Enter everything individually that goes into the recipe, then weigh whatever portion you eat as a percentage of the whole.

Did you know there are 16 tbsp in one cup... if you have say 2 cups that's 32 ... if your serving size is 2 tbsp you would 16 total servings in a 2cup recipe. .. measure the works then divide and conquer.