I bring a large pot of generously salted water to a rolling boil first. Add the spaghetti and cook according to the package directions until just al dente. Drain it in a colander, then toss with a touch of olive oil so it doesn't stick together while you work.
While the pasta cooks, I heat the marinara sauce in a saucepan over medium heat, stirring occasionally. You just want it warmed through and glossy, not reduced or thickened too much. This sauce becomes the "blood" your monster sits in, so keep it saucy.
I spread the plain spaghetti out on my serving platter first, shaping it into a rough mound or face outline. Then I spoon the warm sauce over the center, letting it pool and drip toward the edges like something out of a horror movie.
Slice the mozzarella into two rounds and set them where the eyes should go. I cut the tips off an olive to make two rings, then place one ring in the center of each mozzarella circle for the pupils. A dab of tomato passata around the edges gives that gross, bloodshot finish.
I slice olives thinly for eyebrows and place them just above each eye. For the mouth, I arrange more olive slices side by side beneath the eyes. Cut small mozzarella triangles for teeth, plus two longer pointed ones for fangs, and tuck them into the "mouth" area.
I always add one last creepy detail: a spider made from a halved olive as the body, with thin olive strips fanned out as legs. Place it near the eyes or crawling across the sauce for extra spookiness right before serving.