Cut the guanciale or pancetta into small strips or cubes. Add to a cold skillet and turn heat to medium-low. Render slowly, 5-7 minutes, until crispy and the fat is translucent. Remove from heat.
While the pork renders, whisk the eggs in a large bowl. Add the grated cheeses (reserve a little for garnish) and the cracked black pepper. Mix until thick and paste-like. Set aside.
Bring a large pot of salted water to a boil. Cook the tortellini according to package directions (fresh: 2-4 min, frozen: 4-6 min, dried: 7-9 min). Before draining, reserve 1.5 cups of the starchy pasta water. Drain the pasta.
Take the skillet with the crispy pork off the heat. Add about 1/2 cup of the hot pasta water to the pork fat and swirl. Immediately add the drained tortellini to the skillet and toss to coat.
Slowly drizzle 1/2 cup of the hot pasta water into the egg and cheese mixture while whisking constantly. This tempers the eggs so they don't scramble when added to the pan.
Pour the warmed egg mixture over the hot tortellini in the skillet. Toss and stir vigorously. The sauce will become creamy and glossy in about 30 seconds. If too thick, add more pasta water; if too thin, add more cheese.
Serve immediately in warmed bowls. Top with reserved cheese, an extra crack of pepper, and a pinch of flaky salt if desired. Eat right away.