As an allotment grower I am envious of your climate. Peas are very good nitrogen fixers and grow fairly quickly tho you will need to give them some kind of mesh to climb up. If the soil has been exhausted of nutrients you will need to start adding some back. Growing a green manure like mustard is a good way of adding back nitrogen. Simply sow mustard seed and let it grow then dig it in just as it starts to flower, it will add back nutes as well as organic structure to the soil << also tasty in salads.
If your soil underneath your 'dump' is very compacted and hasn't been cultivated for years potatoes make a good first crop - this is just because it forces you to do some heavy digging to get the buggers up - and breaking up heavy soil helps aeration for your next crop of whatever lol.
If you want to grow any roots (like carrots or parsnip etc). They prefer a sandy soil so adding a bit of sand to the mix would help those. Here we get what we call carrot root fly - the grubs burrow into the roots introducing holes and rot - if this is an issue in your area you would need to create screens about 3' high to stop the root fly infecting your plants.
But personally I would start off creating a herb garden (unless food production is a priority). Here fresh herbs are expensive at the markets (compared to most fruit n veg) and a nice bunch of fresh herbs can lift any dish you cook from mundane to super tasty.
Also look at what is commercially grown in your region - these will always do well (but may be so cheap at the markets to make them not worth the effort). So try and find things you like that are tasty treats.
If you are considering fruits do remember that it can take a couple of years for flowering bushes to mature to fruiting and that fruit trees can take considerably longer from saplings. Tomatoes, Peppers and Chilli plants or Peppers usually do well in warmer climates but can take a lot of watering and nute feeding to do well and be prone insects like leaf miners as well as aphids which can carry mosaic virus.
Oh and as you start your veg garden you will find that creating a compost heap that you can use helps (we have two - one is constantly being added to and the other digesting - rotated every year). Into this you can add all your plant trimmings - vegetable food peelings and cannabis waste trims - all makes for adding more nutrients (for free) for the next year.
Happy gardening
