Gently wash produce under cool running water, pat dry, and prepare as directed.
Place the olive oil, vinegar, mustard, minced garlic, salt, and pepper in a small jar with a tight-fitting lid.
Place the lettuce in a large serving bowl. Layer on the microgreens, red onion, tomatoes, radishes, cucumber, and avocado.
When ready to serve, shake the dressing vigorously to combine. Taste and adjust the seasoning.
Pour the dressing over the salad and toss gently. Do not oversaturate.
Serve immediately. If not serving right away, refrigerate the salad and the dressing separately and combine just before serving. Refrigerate any leftover dressing.