In a medium saucepan, melt butter over medium heat. Add finely chopped onion and cook, stirring occasionally, until softened and translucent, about 5-7 minutes.
Meanwhile, in a small bowl, whisk miso paste with ½ cup of chicken broth until miso is fully dissolved and smooth.
Add Great Northern beans to saucepan with the softened onion. Pour in the remaining 1 cup of chicken broth and dissolved miso mixture. Stir well to combine.
Bring to a gentle simmer. Cover and reduce heat to low; cook for 10-12 minutes, allowing the flavours to meld and the beans to warm through.
Stir in the fresh spinach. Cook another 2-3 minutes, stirring gently, until the spinach has wilted.
Serve the Miso Beans warm with toasted crusty bread, if desired.
