Prepare the Vinaigrette: Add the fresh lemon juice, Dijon mustard, fresh thyme, kosher salt, and olive oil to a mason jar. Seal and shake well until the dressing is combined. Alternatively, place the lemon juice, mustard, thyme, and salt in a small bowl and whisk in the olive oil until well combined. Set aside. Prepare the Asparagus: Trim the ends from the asparagus, then slice the stalks into 2–3-inch pieces.
Blanch the Vegetables: Bring a pot of water to a boil, add the asparagus and peas, and cook for 1 minute.
Stop the Cooking: Immediately transfer the asparagus and peas to a bowl of ice water to blanch them and stop the cooking process.
Dress the Vegetables: Transfer the blanched asparagus and peas to a bowl. Pour in the prepared vinaigrette and toss until the vegetables are evenly coated. Arrange the Lettuce: Spread the lettuce over a serving platter, or divide it evenly among 4 plates.
Assemble the Salad: Spoon the dressed asparagus and peas over the lettuce, then finish with fresh mint, crumbled goat cheese, and a sprinkle of sunflower seeds.
Serve: Serve the salad as desired.