Make Vinaigrette: In a medium bowl or jar, combine lemon juice, vinegar, sugar, coarse salt, and ground pepper. Whisk, or cover and shake, until salt and sugar have dissolved.
Add oil; whisk or shake until thoroughly combined. Refrigerate in an airtight container up to 2 weeks.
Fill a medium bowl with ice water; set aside. Set a steamer basket in a large saucepan with a lid. Fill with enough salted water to come just below basket; bring to a boil. Place green beans in basket, and reduce heat to a simmer. Cover pan; steam until tender, 4 to 6 minutes. Using tongs or a slotted spoon, transfer beans to ice water to stop cooking; drain and pat dry with paper towels.
Place on a serving platter. Top with shallot, and drizzle with vinaigrette; season with salt and pepper.