Preheat oven to 400 degrees. Heat the oil in a large pot with an oven-proof lid over medium-high heat. Add the onion and sauté for 2-3 minutes. Turn the heat to medium and add the bacon if using. Continue to sauté until the onion is translucent and the bacon is cooked but not crispy. Add the farro or barley and the lemon juice and turn the heat to high. Cook until the farro or barley absorbs the juice. Add the chicken broth and bring to a boil. As soon as it boils, set the lid on the pot and put it into the oven. Bake for 30 to 35 minutes until the farro or barley is just tender. Remove from the oven and add the peas and asparagus. If the farro or barley looks dry, add 1/4 cup of hot water or broth. Give it all a good stir and put it back into the oven for another 7 to 8 minutes until the vegetables are just tender (keeping in mind they will continue to cook a little once out of the oven). Remove from the oven and add mint, salt, and Parmesan. Serve immediately.