In a medium soup pot, heat the EVOO, three turns of the pan, over medium. Add the leeks, potatoes, carrots and garlic; season with salt and pepper. Stir 1 minute, then cover and cook, stirring occasionally, until the vegetables soften, 6 to 8 minutes.
Add the stock, peas and tarragon. Simmer until the peas are just tender, 3 minutes for fresh or 5 minutes for frozen. Add the lettuce and stir until just wilted, about 30 seconds. Stir in the lemon juice. Serve with the crusty bread.