The pepper is not that hard. Take the pepper and cut off the top and bottom. You should be able to remove the core. Cut the side and unroll the flesh, remove any inedible parts left and dice it. That is the way I do it. I either use the top and bottom for stocks, or I just cut them up and use them in the recipe.
The best tip I have for onions is to cut them in half and then peal them. Hold the onion in place by resting our hand on top, keeping your hand flat. make three horizontal cuts on the stem side and then make some vertical cuts about 1/4" depending on recipe. Then you just slice into dices. Make sure your knife is sharp and take your time. I would hit youtube up for a video demonstration, or there might be some on this site.
It might be easier to just slice the onions and then chop the slices.