Heat oil and butter in large pot over medium heat. Add cabbage and/or kale, and leeks, to pot and stir until well combined. Cover pot and cook 8 to 10 minutes or until greens are tender, stirring occasionally.

Stir in potatoes, thyme sprigs, bay leaf, stock and salt. Heat to boiling over medium-high heat, then reduce heat to medium-low, cover and cook 14 to 16 minutes or until potatoes are very tender. Remove thyme sprigs and bay leaf and discard.

If desired, remove from heat and use an immersion blender to puree some of the soup while still leaving a lot of it chunky (you can also do this by transferring about half of the soup to a blender or food processor). 

Return soup to medium-low heat and stir in half and half, lemon juice, pepper and nutmeg. Serve immediately, garnished with bacon and sliced leeks, if desired.