Applying heat to the dressing allows the shallots to mellow and the mustard and rosemary to steep in the olive oil.
Heat olive oil in a small skillet over medium-low. Add minced shallot; sauté 4 minutes. Remove pan from heat; stir in vinegar, mustard, rosemary, salt, and pepper. Cook haricots verts according to package directions; toss with shallot mixture.