Melt 3 tablespoons butter in a large skillet over medium-high heat. Add the chopped fennel, garlic, rosemary and a pinch each of salt and pepper. Cook, stirring occasionally, until the fennel is softened, about 5 minutes. Stir in the barley until combined.
Add the chicken broth and 2 cups water. Bring to a simmer, then cover, reduce the heat to medium low and cook until the barley is tender but still a bit saucy, 8 to 10 minutes.
Uncover and stir in the shrimp and peas. Cover and cook, stirring occasionally, until the shrimp are just cooked through, about 5 minutes. Remove from the heat and stir in the lemon zest and juice and the remaining 2 tablespoons butter until melted. Stir in the parmesan and 2 tablespoons fennel fronds; season with salt and pepper. Divide among bowls and top with the remaining fennel fronds.