Preheat the oven to 400°F (200°C).
In a 2- to 4-quart saucepan, bring the water, butter, and salt to a simmer over medium heat. Add the flour all at once and stir vigorously until the mixture masses and detaches itself from the sides of the pan. Reduce the heat to low and cook, beating constantly, until the batter is very stiff and almost shiny, usually a few minutes.
Off the heat, add the eggs one by one, beating thoroughly with a wooden spoon to completely incorporate each egg before adding the next. The mixture will initially resist each addition; you'll find yourself cutting through and slapping together slabs of slippery, warm paste until it gradually absorbs the egg and becomes sticky again. The final mixture should be no hotter than tepid. Add the pepper to taste and stir in the Gruyère.
If you are proficient with one, transfer the batter to a pastry bag, and pipe 2-to 3-inch-long bands onto a parchment paper-lined (or nonstick) baking sheet about 2 inches apart. Otherwise, use a spoon to scoop out a heaping tablespoon of batter per gougère and a second spoon to scrape it into a peaky mound on the prepared baking sheet.
Bake until firm and a rich golden brown, about 25 minutes. Inevitably, some bits of cheese will ooze and form a delicious, crispy bib on the edges of the gougeres. To check doneness, remove 1 gougère and pry open. The interior strands of dough should be tender and moist, but not mushy; if they are, close the gougère and return it to the oven to bake with the rest for another few minutes. If you are concerned they may over brown, simply turn off the oven and leave to finish cooking in the ambient heat.
Meanwhile, cut the bacon into 1 1/2- to 2-inch segments and panfry or roast to your taste. Drain on towels.
Serve the gougère warm from the oven (or reheated), split through the middle and overstuffed with a few pieces of bacon, several leaves of peppery arugula, and a few ringlets of the pickled onions.