Step 1Trim any tough dark green tops off the leeks, keeping the tender green parts. If the leeks are thick, cut them lengthwise in half. Cut into 1-inch pieces crosswise, then wash well and dry well. Sprinkle the shrimp with salt and pepper.
Step 2Heat a large skillet or wok over high heat until a drop of water skitters across the surface. Add the oil and carefully swirl to coat; it should be shimmering. Add the leeks, sprinkle with salt, and cook, stirring occasionally, until browned in spots, 1 to 2 minutes. Add the chiles and cook, stirring, for 1 minute. Add the shrimp and cook, stirring occasionally, until lightly browned and opaque throughout, 3 to 4 minutes. Remove the skillet from the heat.
Step 3Fold in the watercress; it will wilt in the residual heat. Transfer to a serving plate and garnish with the watercress tops and flowers. Squeeze lemon juice on top and serve.