To make the thyme croutons, preheat the oven to 40Cut the bread slices into cubes and, in a bowl, toss with the olive oil, fresh thyme, and sea salt. Spread into one layer on a sheet pan. Bake until the bread is golden and toasted, about 10 minutes. Allow to cool slightly or completely before adding to the salad.
Combine all dressing ingredients in a small jar. Seal and mix vigorously until creamy. Taste and adjust salt if needed.
To make the salad, first roast the leeks and fennel. Preheat the oven to 40Slice the leeks in half lengthwise, then into ½-inch pieces and place into a large bowl. Trim the fronds from the fennel and reserve. Cut the bulb in half lengthwise, remove the core and tough outer layers, then slice lengthwise.
Add to the bowl with the leeks. Drizzle with olive oil and season with thyme leaves and sea salt. Spread into one layer on a sheet pan and roast until caramelized, a total of 20-25 minutes with one flip midway. Allow to cool slightly or completely.
In a large bowl, add spring greens and sprouts. Toss the parsley leaves into the salad with the reserved fennel fronds. Slice the snap peas in half lengthwise and toss into the bowl. Trim and slice the radishes into wedges and add to the bowl.
Toss the thyme croutons and roasted vegetables into the large bowl of salad greens. Add the dressing and toss. Let the salad rest for about 15 minutes to allow the croutons to soften slightly and the flavors to marry. Divide into bowls or plates and sprinkle each with Celtic Sea Salt.