Heat the olive oil over medium heat in a deep saucepan. Add the onions and cook, stirring often until they soften and become translucent, for 4 – 5 minutes.
Add the drained beans, stock, and sage. Bring to a boil. Reduce heat and simmer the beans for 20 minutes. Season with salt and pepper.
Remove the sage leaves. Use a wooden spoon to crush the beans into a coarse purée. Add more olive oil to create a spreadable mixture.
Spoon some of the bean mixture onto each crostino mounding it slightly. Drizzle with extra virgin olive oil. Season with freshly ground black pepper. Place a few leaves of fresh basil on each crostino.