Place lentils in a medium saucepan filled 2/3‘s with water, and bring to a boil over medium-high heat, stirring occasionally. Reduce heat to medium-low, and simmer until the lentils are just tender (about 15 minutes). Start checking for doneness around 13 to 14 minutes, because if you overcook the lentils, they will turn to mush.
While the lentils are cooking, add 2 cups watercress, parsley, olive oil, red wine vinegar, garlic, a few pinches of salt and several turns of black pepper to a blender or food processor. Blend until smooth. Drain the lentils, then transfer to a large bowl. Toss hot lentils with most of the watercress dressing (reserve a little bit for drizzling at the end). Season to taste with additional salt and pepper.
Bring a large pot of salted water to a boil. Drop in asparagus and fiddlehead ferns, and cook just until the asparagus are tender (2 to 3 minutes). Drain asparagus and ferns, then cut the spears into 2-inch long segments. Toss asparagus, ferns and remaining watercress with the dressed lentils.
To serve, divide lentil salad between 4 plates. Top each with 1/2 of a ball of burrata and a drizzle of remaining dressing. Finish with a sprinkling of flaky sea salt and black pepper.