Steam the beans in a large covered pot fitted with a steamer basket over 1/2” of water until crisp-tender, then plunge beans into a bowl of ice water to stop cooking. Drain and pat dry with towels.
In a small jar fitted with a lid, shake together oil, lemon zest, lemon juice, shallots, thyme, salt and black pepper.
Arrange mesclun greens on a platter, and top with beans and red pepper. Shake the dressing well, then add the crème fraîche and shake again. Pour dressing over salad, top with blue brie and serve immediately with baguette slices, if desired.Do not add the sour cream or crème fraîche until ready to serve, or the dressing may curdle.