You can use pasteurized eggs for the aïoli, if you like. Also, if you’re slicing the pancetta by hand, freezing it briefly first will firm it and make it easier to slice.
Using a mortar and pestle or the flat side of a chef’s knife, mash the garlic to a paste with a pinch of salt. In a medium bowl, whisk the egg yolks and half of the mashed garlic. Whisk in the oil a few drops at a time. As the mixture begins to emulsify, add the oil in a slow stream. When the aïoli becomes too thick to whisk (after you’ve added about half the oil), whisk in 1 tsp. of the lemon juice to loosen it. Continue to whisk in the oil in a thin stream. If the aïoli thickens too much before you finish adding the oil, loosen it with a drop of water. Taste and adjust the seasoning with salt and the remaining garlic. If the aïoli tastes a little flat, add a little more lemon juice. The aïoli should be thick and spreadable, like mayonnaise, so don’t thin it too much.
Position a rack 6 inches from the broiler element and heat the broiler to high. Arrange the pancetta in a single layer on a rimmed baking sheet and broil, flipping as necessary, until it's golden brown and crisp, about 8 minutes total.
Meanwhile, spread the aïoli on the toasted bread. Peel and slice the avocado; arrange a few slices on each of four slices of toast, pressing the avocado onto the bread. Season with a pinch of salt. Lay a few slices of tomato on top of the avocado and season again with a pinch of salt. Put three pieces of pancetta on each sandwich and top each with a small handful of arugula. Top with the remaining slices of toast, cut the sandwiches in half, if you like, and serve immediately.