Bring the barley, 3 1/2 cups water and 1/2 teaspoon salt to a boil in a medium saucepan over high heat.
Reduce heat to low, cover and simmer until the barley is chewy but tender, about 50 minutes.
Meanwhile, add the bacon in a single layer to a large nonstick skillet over medium heat and cook, stirring occasionally, until golden brown and crisp, about 6 minutes.
Remove the skillet from the heat and transfer the bacon with a slotted spoon to paper towels to drain; reserve the bacon drippings in the skillet.
Return the skillet to medium heat and melt the butter in the bacon drippings.
Add the onion and 1/4 teaspoon salt and cook, stirring occasionally, until deep golden brown, about 25 minutes.
Add the peas and stir until heated through, about 1 minute.
Drain any excess liquid from the barley.
Stir the barley into the onion-pea mixture.
Stir in the dill, lemon juice and 1/2 teaspoon each salt and pepper.
Transfer to a serving bowl and sprinkle with the bacon.