1. You can just use all onion or garlic powder, if you only have one of them. Fresh garlic is not the same, however, as a last resort substitution, you can use 3 garlic cloves, minced, instead of onion & garlic powder. Saute in the heated oil before proceeding with the recipe.
2. Tomato passata is pureed plain tomatoes that is not seasoned. Unlike crushed tomatoes, it is thick and smooth, like a thick tomato soup. In Australia it can be purchased in a bottle in the pasta aisle of major supermarkets (Coles, Woolworths). You can make your own simply by using a blender to puree canned tomatoes.
America: If you can't find passata in the US (it is available at Walmart, Costco), a great substitute is US canned tomato sauce eg Hunts (yes, I'm such a food nerd I brought back cans to use for this recipe and it's a near perfect substitute).
3. Black beans are commonly used in Mexican & Tex Mex cooking. It's becoming more widely available in Australia. It can be purchased in the Mexican or canned beans aisle in most major supermarkets, as well as Harris Farms and Thomas Dux. You can substitute with red kidney beans or another type of beans if you prefer.
4. The refried beans should loosen as it heats and stir through easily. If not, add a splash of water.
5. To bake covered, I just pop a baking tray on top on the baking dish. Otherwise, cover with foil.
6. To make ahead, roll up enchiladas and place in casserole dish (can smear with sauce underneath). Keep sauce separate and top just before baking.
7. Nutrition per serving, assuming you use lean beef mince (this is a great recipe for using lean beef, you would never know!).