Add butter to a deep skillet over medium heat.
Add onion and garlic and cook for 8-10 minutes, until softened.
Add all other ingredients except for the spinach to the skillet. Stir to combine.
Partially mash about ¼ the beans with a potato masher.
Bring to a simmer, being careful not to boil. Simmer for 10-15 minutes.
Turn heat to low and fold in spinach, stir until the spinach is wilted.
Squeeze in lemon, stir again and turn off heat.
Divide out, add your favorite toppings, and serve with toasty bread.
