Heat oil in a heavy-bottomed saute pan on medium-low heat. Add onion and garlic. Cook on medium-low heat for a couple of minutes until onion appears glazed and is starting to brown. Reduce heat to low and cook the onion for a further 8-10 minutes, tossing occasionally until translucent and gently caramelized.
Add mustard, butter beans, smoked paprika and balsamic vinegar. Increase heat to medium and cook for a 2 minutes until heated through. Add the vegetable stock. Mix well and cook for a further 2-4 minutes until some of the stock has evaporated.
Add spinach. Mix well. Cook for a minute until spinach is wilted and remove from heat. Serve hot or cool completely and store in the fridge in an air-tight lidded glass container for up to 2 days.
Butter Beans - You can replace them with any canned legume in the pantry. Although butter beans taste the best in this recipe, other good substitutions are cannelini beans and chickpeas.