Cook the bacon in a 12-inch skillet over medium-high heat until browned and crisp. Remove from the skillet, drain on paper towels, and crumble into small pieces.

Increase the heat to medium high. Sprinkle the shrimp with 1/4 tsp. salt and a few grinds pepper. Working in two batches to avoid crowding, cook the shrimp in the bacon fat until lightly browned and opaque, 1-1/2 to 2 minutes per side. Transfer the shrimp to a plate.

Add the shallots or onions to the skillet and cook until they start to soften, about 1 minute. Add the barley and stir until coated with the bacon fat, about 30 seconds. Add the lemon juice and cook, stirring, for 15 seconds. Pour in the chicken broth and bring to a boil, scraping up any browned bits in the pan. Reduce the heat to medium low, cover, and simmer until the barley is tender, 12 minutes. Uncover the pan, raise the heat to medium high, and cook, stirring occasionally, until most of the liquid has evaporated, 1 to 2 minutes. Stir in the spinach and cook until wilted, 1 minute. Stir in the bacon, shrimp, Pecorino, and lemon zest and heat through, 1 to 2 minutes. Season to taste with salt and pepper and serve immediately.