Heat a fireplace to medium heat. Adjust the grill racks over the flame and heat a heavy Dutch oven over the rack. Alternatively, heat a medium saucepan over medium heat.
Once hot, add the oil to the saucepan. Add the pancetta and render some of its fat. Add the rosemary and garlic, and saute until fragrant. Stir in the beans, lemon juice and some salt and pepper. Mash the beans with the back of a wooden spoon. Don't mash them all smooth, you want some texture.
Toast the baguette slices. Serve the beans on top of the toasted baguette. Drizzle with olive oil if desired.
Recipe courtesy Gabriele Corcos and Debi Mazar, Extra Virgin, 2011