Take the pan off the heat before the cheese goes in, grate it finely, and stir as little as you can afterwards. Cheese held above its melting point clumps and pushes its fat out, which is the grainy sauce.
The teaspoon of lemon juice is not a garnish. The acid keeps the cheese proteins from binding together, the same way wine does in a fondue. If the sauce does tighten, another squeeze will usually bring it back.
Keep the sauce looser than looks right when you assemble it. The pasta goes on drinking liquid in the oven.
Toast the topping separately and add it at the table. Scattered on raw before baking it softens in the steam.
Leftover puree keeps four days in the fridge and three months frozen. More than two thirds of a cup in the sauce and it starts tasting sweet rather than cheesy.
Keeping it: three days in the fridge, three months frozen. Reheat covered with a splash of milk, and make the topping fresh.