Warm broth in a medium saucepan over low heat until ready to use. Heat the oil in a large heavy bottomed pot or Dutch oven over medium-high heat. Add pancetta and cook until crisp, about 10 minutes. Remove pancetta to a paper-towel lined plate and reserve. Return pot to stove, add the leeks, season with 1/2 teaspoon of salt and pepper, and cook until softened, about 5 minutes more.
Add the farro and stir to coat. Add white wine or vermouth until alcohol smell is cooked off. Stir in 3 cups of the broth or water and bring to a simmer. Cover and simmer, stirring every 8 to 10 minutes, until rice is tender and creamy but the center of each farro grain is tender to the bite (the mixture should be soupier than regular risotto), about 20 to 25 minutes. Stir in the asparagus and peas and cook until bright green, about 5 minutes more.
Turn off the heat, stir in the butter and the Parmesan, cover and rest a few minutes before serving. Just before serving, add the additional broth (if desired) then stir in half of the herbs. Garnish with reserved pancetta, remaining herbs, a drizzle of olive oil, and, if desired, a squeeze of lemon juice.