Prepare the spinach: Wash the spinach, break off the thick stems. Blanch in boiling salted water for 2–3 minutes, then transfer to cold water. Squeeze out the water thoroughly.

Sauté the onion: Slice the onions into half-moons. Heat the olive oil in a pot, and cook the onions over low heat for 8–10 minutes until softened, without browning.

Add tomatoes: Add the tomatoes and cook for 5 minutes until their water evaporates. Add salt.

Add spinach: Add the squeezed spinach and mix. Cook together for 3–4 minutes, then turn off the heat.

Serve: Allow it to cool. Once cooled, squeeze in the lemon juice. Serve at room temperature or chilled.