Put the washed scallops on a paper towel and pat dry, then season on all sides with pepper. Let the scallops rest while they come to room temperature—about 10 minutes.
Meanwhile, get a big pot of salted water on the stove, heat to a boil and cook the fettuccine according to package instructions. Drain, set aside and keep warm.
In a medium saucepan, heat the butter and add the garlic and onion. When the garlic and onion are softened, pour in the heavy cream and stir until warm enough to melt the Parmesan. Toss in the Parmesan and stir until it’s incorporated.
Spoon in the adobo liquid, to taste, and keep warm. (You can also add the diced chipotle peppers for a stronger flavor.) Meanwhile, heat the 2 tablespoons of butter in a large skillet over medium-high heat until the butter turns brown and smells nutty.
Add the scallops to the hot butter and cook until deep golden brown on 1 side—about 1 or 2 minutes, depending on size. Flip the scallops and spoon the butter over them repeatedly until the second side is browned—about 1 more minute.
Just before serving, stir the cilantro into the sauce. Toss the pasta with the cream sauce and plate. Top the pasta with the seared scallops.