Zest 1 medium lemon, and juice half of the fruit. Set aside.
Meanwhile, melt 4 tablespoons unsalted butter in a large skillet or Dutch oven over medium-low heat.
Add the zest, half of the lemon juice, and ¾ teaspoon freshly ground black pepper to the skillet. Stir frequently, about 1 - 2 minutes, until the black pepper is slightly toasted and aromatic and the lemon juice has reduced slightly. Reduce the heat to low.
Once the pot of water has come to a boil, season generously, and add 1 batch ricotta gnocchi.Once they float to the surface, boil for another 30 seconds (in total, they should take about 1 - 3 minutes to cook).Remove immediately with a slotted spoon and transfer to the skillet, reserving about ½ cup of pasta water.
Add ¼ cup of the pasta water to the skillet, and increase the heat to medium-low. Let the sauce reduce for a minute, occasionally stirring the gnocchi.
Turn the heat off, then slowly sprinkle ¾ of the parmigiano-reggiano cheese evenly over the gnocchi, stirring constantly until the sauce begins to emulsify, appear creamy, and clings to the gnocchi. Add any additional pasta water if the sauce is too thick.
Note: If the sauce is not thickening, you can turn the heat on the lowest setting to help it thicken slightly. Make sure to keep vigorously stirring or the sauce won’t emulsify properly.
Taste the sauce, and adjust seasonings as needed; season with salt, pepper, and/or additional reserved lemon juice if you want it more tart. The pepper should really come through, as it will help balance the acidity of the lemon juice and add a bit of heat, so I like to give the dish a generous grind at the end.
Garnish with the remaining cheese and herbs. Serve immediately.
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Notes
The trickiest part of the sauce is to create a homogenous, creamy emulsion. Make sure to add the cheese slowly, whisking constantly, on low heat. If you find that the sauce doesn't emulsify and the oil begins to separate, you can add a bit more pasta water (or even a touch of cold cream) to help bring the sauce together.
Variation: For a creamier sauce, add ½ cup heavy cream with the lemon juice, zest, and black pepper. Let reduce until thick enough to coat the back of a spoon, then proceed with the recipe as instructed.