"I love a good Nicoise salad, and the potatoes, green beans, olives, tomatoes, and chopped egg are perfect for layering. The salad amounts listed are just estimates and can be tweaked to your individual taste."
Bring a pot of salted water to a boil over medium-high heat. Add green beans and cook for just 2 minutes. Transfer to a bowl of ice water using a slotted spoon.
Transfer potato cubes to the same pot; simmer over medium heat until tender, but not soft, 5 to 8 minutes. Remove potatoes with a slotted spoon and drain on a paper towel-lined plate. Slide out paper towel and drizzle warm potatoes with about 1 tablespoon olive oil.
Drain green beans. Cut into 1/2-inch to 3/4-inch pieces.
Place the potatoes in a 1-quart canning jar; even out the layer. Add in this order a layer of chopped green beans, tuna, tomatoes, olives, eggs, and anchovy fillet, evening out layers as you go. Top with chopped parsley. Leave about 1/2 inch space at the top. Close with lid and refrigerate while you make the dressing.
Place mustard and anchovies in a mixing bowl. Add vinegar, lemon juice, shallots, tarragon, and avocado, olive oil, black pepper, and cayenne pepper. Blend with an immersion blender until thoroughly emulsified, 1 or 2 minutes. If dressing seems too thick, stir in 1 or 2 teaspoons water or lemon juice.
Pour about 1/4 cup dressing into salad jar, or more to taste. Toss with a fork. Or you can shake the jar and transfer dressed salad to a bowl.