Preheat the oven to 425 degrees F. Line a large rimmed baking sheet with parchment paper.
Toast the pine nuts in a saute pan over medium-high heat until golden in spots, 3 to 5 minutes. Transfer to a bowl and reserve.
Arrange the pancetta in a single layer on the prepared baking sheet. Bake until golden brown and crisp, about 10 minutes. Transfer to a paper-towel-lined plate to cool.
Combine the toasted pine nuts, garlic and Parmesan in a food processor; pulse to finely chop. Add the arugula and continue to pulse until finely chopped. With the motor running, slowly add the olive oil; process until incorporated. Add the lemon zest, lemon juice, 1/4 teaspoon salt and a few grinds of pepper, and pulse to combine.
Spoon a nice dollop of pesto onto each slice of baguette, top with a pancetta crisp and serve.
Any remaining pesto can be kept refrigerated for up to 2 days in an airtight container with plastic wrap pressed directly to the surface of the pesto.