Pour 4 cups water into a medium saucepan. Add 1 teaspoon of the kosher salt and set the saucepan over high heat so the water comes to a boil.

While the water heats, finish the small prep. Chop the onion, slice the garlic, zest the lemon, and squeeze the juice from one lemon half into a small cup. Cut the other lemon half into wedges for serving.

Set a large skillet over medium heat and add the olive oil.

Add the chopped onion to the skillet. Cook for 4 to 5 minutes, stirring now and then, until the onion looks soft and glossy.

Add the sliced garlic to the skillet. Cook for about 30 seconds, stirring, just until it smells fragrant.

Add the cherry tomatoes to the skillet with 1/4 teaspoon of the remaining salt. Cook for 6 to 8 minutes, stirring occasionally, until many of the tomatoes split open.

Press some of the softened tomatoes against the side of the skillet with a spoon. Keep cooking until the released juices make a chunky sauce in the pan.

Add the spinach to the skillet. Stir for 1 to 2 minutes, until the leaves wilt down into the tomatoes.

Add the drained tuna to the skillet. Sprinkle in the lemon zest and black pepper, then add 1 tablespoon of the lemon juice.

Gently fold the tuna into the tomato mixture for 1 to 2 minutes, just until the tuna is heated through and coated in the sauce. Turn the heat to low to keep it warm.

When the water in the saucepan is boiling, slowly pour in the quick-cooking polenta while whisking the whole time. This keeps the polenta from forming lumps.

Reduce the heat under the saucepan to low. Cook the polenta for 4 to 5 minutes, stirring often with a spoon, until it is thick and smooth.