Brazilian Escondidinho de Mandioca

From the garden

• 2 lb cassava roots (yuca), peeled deeply, cored and cut into chunks · weight after peeling and coring; sweet table varieties only
• 1 medium yellow onion, finely chopped
• 3 cloves garlic, minced
• 2 medium ripe tomatoes, chopped
• ¼ cup fresh parsley, chopped
• 3 green onions, thinly sliced

From the pantry

• 1 lb ground beef
• 1 tbsp tomato paste
• 3 tbsp butter · for the puree, plus a little for the dish
• ½ cup whole milk · warmed
• 1 cup shredded mozzarella · for the top
• salt · to taste
• black pepper

1. Peel each root right through both layers, the brown bark and the thick pinkish layer beneath it, since that inner layer holds most of the cyanogenic compounds. Split the flesh lengthwise, pull out the woody central core, and cut the rest into 2-inch chunks so the cooking can reach the middle.
2. Soak the chunks in a bowl of cold water for at least an hour, changing the water once. Sweet table cassava does not strictly require it, but the soak is a sensible extra margin and can be done the night before.
3. Boil the drained chunks in fresh salted water until a fork meets no resistance at all, 25 to 35 minutes, then drain and pour the cooking water away rather than using it for anything. Any piece that tastes bitter or numbs the mouth means the whole batch gets thrown out, cooked or not.
4. Mash the hot cassava with the butter and warm milk until smooth, lifting out any stringy fibers that surface. Season firmly with salt and pepper, because the puree should taste finished on its own.
5. Brown the ground beef in a wide skillet over medium-high heat, breaking it up, then add the onion and cook 5 minutes until soft. Stir in the garlic and tomato paste for a minute, add the tomatoes and a good pinch of salt, and cook 8 minutes until they collapse into a thick filling.
6. Spread the beef in a buttered baking dish, cover it with the cassava puree, and scatter the mozzarella over the top. Bake at 400°F for 20 minutes, until the cheese is browned in patches, then scatter the parsley and green onions over just before serving.