Cook the pasta in salted boiling water for 1-2 minutes shorter than the package instructions.
Heat the olive oil in the large skillet over medium-high heat. Add onion, if using, and cook until onion just starts to take on color. Lower the heat to medium and add garlic to pan; cook for 1 minute.
Time this step so that cooking finishes just as the pasta is ready. You don't want to overcook your garlic.
When the pasta has cooked for 1-2 minutes less than the package instructs, use the spider to transfer it into the skillet with the oil and garlic. Add a small splash of pasta water to the pan, reserving the rest of the pasta water in case you need it later.
Sprinkle the cornstarch over the skillet ¼ teaspoon at a time, stirring to combine before adding the next spoonful. If the mixture becomes too sticky, add a small amount of pasta water and stir.
Sprinkle the yeast over the skillet a tablespoon at a time, stirring to combine between spoonfuls.