For the French vinaigrette (Makes about 1 1/2 cups):
1 cup canola oil
1/4 cup white vinegar
1/4 cup cider vinegar
2 teaspoons Dijon mustard
1 large garlic clove, smashed
1/4 teaspoon mustard powder
1/4 teaspoon lemon-pepper spice mix (see Recipe Note)
Kosher salt and freshly ground black pepper
For the salad:
1 pound fingerling potatoes
8 ounces green beans or haricot verts
3 tablespoons unsalted butter, divided (melted or at room temperature)
3 4.5-ounce cans tuna packed in olive oil, drained
4 large hard-boiled eggs, sliced
2 medium ripe tomatoes, quartered
Niçoise olives (optional)
3 cups salad greens
For the dressing, combine the oil, vinegars, Dijon, and garlic in a blender or small food processor and blend until the dressing is thick and creamy (emulsified). Whisk in mustard powder, lemon-pepper seasoning, and a generous amount of salt and pepper. Taste and adjust vinegar, lemon-pepper seasoning, and salt to taste.
This dressing will make more than you need for one salad; the remaining vinaigrette can be refrigerated a few weeks.
For the salad, place potatoes in a large stockpot and cover with a few inches of water. Add a generous amount of salt and bring to a boil. Cook until potatoes are tender and can easily be pierced with knife, about 10 minutes. Remove the potatoes with a slotted spoon and set aside to cool.
Return the water to a boil and add the green beans. Cook until bright green and crisp-tender, 3 to 4 minutes. Remove beans with a slotted spoon (or drain through a colander) and set aside.
Slice the fingerling potatoes crosswise into thin discs, toss with 2 tablespoons of butter, and season with salt and pepper. In a separate bowl, toss the green beans with remaining tablespoon of butter and season with salt and pepper. In another bowl, stir a few tablespoons vinaigrette into the tuna. Taste and add more until desired flavor is reached. Season with salt and pepper.
Spread lettuce onto a serving platter. Arrange potatoes, green beans, eggs, tomatoes, tuna, and olives over lettuce. Serve with remaining dressing on the side.
Recipe Notes:
- Lemon-pepper spice is a mix of lemon zest, black pepper, and salt, sometimes with dried garlic. You can find it in the spice section at most grocery stores, or just add these ingredients on their own, to taste.