solids Question!

Baby is 6 1/2 months and has loved everything I've given him. Peas, carrots, butternut squash, yam, ground beef, apple, banana. We are avoiding the baby cereal route with the iron and would rather find foods with high iron to give him. So my question is what else should I try? And what's the deal with spinach and nitrates? Spinach seems like a great one to try but I've read a lot about waiting because of nitrate levels? Also not sure how much meat to give. We've been doing about an oz a day. Thanks in advance!!!