Most of the iron in spinach is not bioavailable. Veggies generally aren't good at the iron being bioavailable but spinach is especially bad, and lentils are quite good and contain a ton. Apples have excellent bioavailability but comparatively low iron content OTOH you can just stick a nail into them. Or use an iron pan that also helps. And yes diced apples work quite well in lentil stew the lentils need acid anyway, add spices and aromatics, either noodles or a slice or two of good bread and you have yourself a meal.