Upthread Diana mentioned 20-0-0 for broccoli for big heads, but I don’t know about broccolini. We just grow the broccoli and our soil is high in nitrogen so it does well here. We use compost so enriching the soil is always good. I think your biggest factor is the sunlight. The plant looks very healthy. So I would not expect too much from the one plant. I have limited sun in parts of the yard but the garden is OK. I track the sun every few years from spring until fall during the day to find the sunniest spots for flowers. Maybe broccolini does well in a large container if that could go somewhere with more sun. You would have to have a rich soil mix.
Thanks for that info. I forgot. I started with just one plant just to have an experience with it. My sunniest spots would be in the middle of the front lawn someplace. Our backyard is too shaded. I have a spot that could maybe work as a garden area if we cut out several limbs, but that would be a project for later this summer.