6 portobello mushroom caps, each about 4 inches across

4 tablespoons extra-virgin olive oil, divided

Salt and freshly ground pepper to taste

1/2 cup finely chopped onion

1 garlic clove, minced

One 15-to 19-ounce can white kidney (cannellini) beans, drained and rinsed

1 tablespoon store-bought pesto

1/4 cup freshly grated Parmesan cheese

Position a rack in the top third of the oven and preheat the oven to 400 degrees F.  Lightly oil a baking sheet.

Using a dessert spoon, scrape out and discard the dark gills from the underside of each mushroom cap, taking care not to break the cap.  Place the caps on the baking sheet.  Brush the caps all over with 2 tablespoons oil.  Season with salt and pepper.  Bake until the caps are barely tender, about 7 minutes.  Remove the baking sheet with the caps from the oven.  (The caps can be prepared up to 2 hours and stored at room temperature.)

Meanwhile, heat 1 tablespoon oil in a medium saucepan over medium heat.  Add the onion and cook, stirring often, until the onion is golden, about 6 minutes.  Add the garlic and cook until fragrant, about 1 minute.  Add the beans and 1/2 cup water.  Bring to a boil over high heat.  Cook, stirring often, until the water has almost completely evaporated, about 6 minutes.  Transfer the beans to a food processor and puree.  Add the pesto and pulse until combined.  Season with salt and pepper.  Fill the mushroom caps with the bean puree.  (The mushrooms can be filled 1 hour before serving and stored at room temperature.)

Sprinkle the tops of the mushrooms with the Parmesan.  Drizzle with the remaining 1 tablespoon oil.  Bake until the Parmesan has melted, about 10 minutes.  Serve hot.