Most people do single foods as first food, but that's your choice, if doing BLW then it doesn't have to be
I offered avocado as first food. I think other early offerings were cooked carrots/broccoli, bananas, sweet potatoes, Greek yogurt, and clementines. At the beginning they don't really eat much of anything so I didn't worry too much about protein, carb, veg. Now our formula is iron source (if non-heme iron, we add a source of vitamin C) + something (could be fruit, veg, starchy carb, fat, and/or dairy - often a combo of these) - for example, last night he had sauteed black beans and corn, mixed with mashed sweet potato (with ghee) and avocado.
I really like having specific advice to follow, so I used the 30 day plan in this book. The first 10 days are a different vegetable each day, and it tells you exactly how to prepare them as both finger food and puree, then it gradually gets more varied in the types of food. https://amzn.eu/d/auGVrF2
My son's first food was pumpkin. I had leftover from my dog who was having digestive issues.