The easiest way to improve growth of vegetable that grow above the surface of the soil, is to grow them under cover of plastic sheets or glass.
If you have these available now, that’s great. If not, do have a look online, or go and visit one of the big DIY chains to get some ideas, and then go back online and find the most cost-effective versions of whatever you have been looking at.
Bigger greenhouses can be heated, and if you can heat a greenhouse cost-effectively when energy to do so is available, it will mean that you can grow some produce that would normally be seasonal, year-round.
However, the best use for glass or plastic coverings is to secure and keep your food safe from animals, insects or other pests – when you are growing outside.
Again, please do a web search using terms like ‘what to grow under glass’, ‘how to grow under glass’, or ’10 best foods to grow in my greenhouse’.