Make the dressing: In a small bowl add lemon juice, salt and freshly ground black pepper. Mix until dissolved. Then, slowly drizzle olive oil, whisking until it has formed an emulsion. Check if it's properly seasoned.
In a large salad bowl add baby spinach, tomatoes, cucumbers, chickpeas and red onion.
Toss the salad with the dressing, then crumble feta and toss gently.