Crisp the pancetta in a dry pot over medium heat (or bake at 200°C / 400°F for 10–12 minutes). Drain on paper towel, then crumble. Leave the rendered fat in the pot.
Add the butter to the pancetta fat. Once it foams, add the leeks and onion. Lower the heat and cook for 8–10 minutes, until soft and translucent but not browned.
Add the garlic and stir for 1 minute, until fragrant.
Season with salt, black pepper and thyme.
Add the potatoes and stir for 1 minute to coat.
Pour in just enough stock to cover the potatoes (about 450 ml). Add the bay leaf, bring to a gentle simmer, then cover and cook for 15–20 minutes, until the potatoes fall apart easily.
Remove the bay leaf. Blend with an immersion blender in short pulses until just smooth. Don't overblend, or the soup will turn gluey.
Take the pot off the heat and stir in the cream. Thin with extra stock if needed. Season generously with salt, then add the lemon juice.
Ladle into bowls. Top with a swirl of cream, parsley, croutons and crumbled pancetta. Serve immediately.
