Trim the ends of 1/2 English cucumber. Using a sharp knife or mandoline, thinly slice into rounds (about 1/8 inch). If cucumber has thick skin, peel half or fully before slicing. Pat slices dry on paper towel to remove excess moisture.

Prepare heirloom tomatoes

Wash and core heirloom tomatoes. Cut into 1/2-inch wedges or bite-size pieces to yield approximately 1 cup. Remove excess seeds/juice if very wet to prevent watering down the salad.

Slice onion

Peel and thinly slice 1/4 cup red onion (about 1/4 small onion). If raw onion is too sharp, soak the slices in cold water for 5 minutes, then drain and pat dry.

Assemble

Add the cucumber slices, tomato pieces, and red onion to the bowl with the spring mix. Toss gently to combine. Season with 1/2 tsp kosher salt and 1/4 tsp freshly ground black pepper, saving additional seasoning if needed after dressing.

Garnish

Finely snip 1 tablespoon fresh chives and sprinkle over the top of the salad just before serving.

Champagne Vinaigrette

Mince shallot

Finely mince one small shallot to yield about 1 tablespoon. This will add mild onion flavor to the vinaigrette.

Combine acid and mustard

In a small bowl or jar, whisk together 3 tablespoons champagne vinegar, 1 teaspoon Dijon mustard, 1 teaspoon honey, and 1 teaspoon fresh lemon juice until combined.

Add shallot, salt, and pepper

Stir in the minced shallot, 1/4 teaspoon salt, and 1/8 teaspoon black pepper to the vinegar mixture.

Emulsify with oil

Slowly drizzle in 6 tablespoons extra-virgin olive oil while whisking constantly (or add oil to jar and shake vigorously) until the dressing is emulsified and slightly thickened. Taste and adjust seasoning: add more salt, pepper, or honey to balance acidity as desired.

Dress the salad

Right before serving, drizzle approximately 3 to 4 tablespoons of champagne vinaigrette over the assembled salad (start with less; you can add more to taste). Toss gently and evenly to coat the greens and vegetables without bruising them. Serve any remaining dressing on the side.