Place beans and chestnuts in a bowl with cold water and let soak overnight. Following day, peel the chestnuts, cutting on the bottom so the skin can be easily peeled with your hands.
Drain the beans and set aside.
Make sure bowl is inside the Instant Pot. Add a tbsp olive oil in the Instant Pot bowl. Add ½ diced onion and a finely chopped garlic clove. Press Saute and set the time to 4 Minutes. Stir the onion and garlic well, to avoid over-browning. Add diced bacon while the garlic and onion are cooking. Stir occasionally until the 4 Minutes are up.
Once you finish sautéing, add the beans and chestnuts. Then add 3 cups water. Add 1 tsp salt and pinch of pepper.
Close and lock the Instant Pot® lid. Make sure the steam release handle is turned to Sealing. Press Pressure Cook, pressure level High, and set the time to 15 Minutes.
Once the time is up, let the steam release naturally or turn the steam release handle.
Once all the pressure is released, open the lid and serve whole, as it is, or you can mince the mixture in a blender.
To garnish (topping): set aside some split cooked chestnuts + sautéed bacon.