Heat oil in a large deep pot over medium-high heat. Add onions, cook, stirring often, until light golden and tender about 7-8 minutes.
Add garlic; cook, stirring until fragrant, about 30 seconds.
Add spinach and salt; cook, tossing, until spinach wilts, about 2-4 minutes. Add tomatoes; cook, stirring, until tomatoes soften, about 1-2 minutes.
Remove from heat and drain off any extra liquid; stir in feta.