To make the vinaigrette: In a small bowl, whisk together the balsamic vinegar, lemon juice, mustard, salt, and pepper. Slowly drizzle the olive oil into the mixture, whisking to incorporate.
Hard-boil the eggs by placing them in a medium pot and covering them with water. Over high heat, bring the water to a boil. Cook the eggs for 3 minutes, then remove the pot from the heat, cover, and let stand for 15 minutes. Drain the water and place the eggs in a bowl of ice water until completely cool, about 10 minutes. Peel the eggs and slice lengthwise.
Into each of 2 widemouthed glass mason jars, layer in the following order: 2 tablespoons of Lemon-Balsamic Vinaigrette at the bottom followed by half of the cucumber, carrot, eggs, red onion, blueberries, and baby spinach, and 2 tablespoons of sunflower seeds. If you don’t have mason jars, you can use widemouthed glass or plastic containers, in which case add the ingredients in any order that you like but store the dressing separately.