In a jar with a tight-fitting lid combine oil, vinegar, mustard, garlic, salt, and pepper. Place lid on top and tighten. Shake vigorously to combine.
Pour dressing into a large salad bowl and add onion. Let sit for at least 10 minutes and up to 30 minutes.
When ready to serve, cut melon into 8 wedges and cut off the rind. Cut wedges crosswise in half.
Add spinach and tomatoes to salad bowl and toss to coat with dressing. Arrange melon wedges on salad, then sprinkle with avocado, almonds, and goat cheese. Serve immediately.