Whisk the lemon juice, Dijon and shallot together in a bowl with 1/4 teaspoon salt and pepper to taste. Let stand 5 minutes to soften the bite of the shallot. Add the olive oil while whisking to emulsify.
Combine the lettuce, radishes, cucumbers and chives in a salad bowl. Toss with just enough vinaigrette to coat. Serve immediately.