Step 1: Make dressing: Combine olive oil, mustard, lemon juice, and half the honey in a jar or bowl. Whisk or shake until smooth.
Step 2: Adjust taste: Add more lemon juice if you like it tangier. Taste as you go.
Step 3: Prep walnuts: In a small bowl, toss walnuts with the remaining honey until coated.
Step 4: Build the salad: Add spinach to bowls, then layer mango, blueberries, walnuts, and feta on top.
Step 5: Add dressing: Drizzle the dressing over everything.
Step 6: Finish: Add a little black pepper and serve immediately.