Heat olive oil in a large skillet over medium heat.
Add chopped onion and minced garlic.
Cook until the onion is tender.
Add the escarole to the skillet.
Stir until the greens wilt slightly.
If using frozen spinach, allow a few extra minutes.
Pour in the chicken stock.
Bring the mixture to a simmer.
Let it cook for about 5 minutes or until heated through.
Add the cannellini beans to the skillet.
Stir to combine thoroughly.
Season with salt and pepper to taste.
Squeeze fresh lemon juice over the dish.
Give it one final stir and serve hot.