Leeks, romaine, peas, and tarragon combine for the greenest soup you've ever seen.
In pot, stir first 3 ingredients over medium until vegetables soften, about 6 minutes. Add stock; bring to simmer over high. Stir in romaine and peas. Simmer until romaine wilts, about 5 minutes. Stir in lemon juice; season. Divide among bowls. Garnish with crème fraîche and tarragon.