Add the minced garlic, oregano, salt, and pepper to a small bowl or mason jar. Pour in the olive oil, red wine vinegar, and lemon juice. Whisk well or seal and shake to combine, then set aside.
Build the Salad
Add the baby spinach to a large salad bowl. Pile the diced cucumber, chopped bell pepper, halved cherry tomatoes, and thinly sliced red onion on top, then scatter the olives over everything.
Add Feta and Dress
Crumble the feta over the top. Pour the dressing over the salad just before serving and toss gently to coat. Serve right away.
Notes
Dress it last: Spinach wilts if dressed too early, so toss just before serving.
Block feta: Crumble it yourself for a creamier texture than the pre-crumbled tubs.
Make it a meal: Add canned tuna, cooked salmon, grilled chicken, or chickpeas.
Make ahead: Keep the dressing separate and toss when ready; undressed components keep 3 to 4 days.