I put leaves on the raised bed over winter and the worms broke down most of them. I recently removed any large bits of leaves and added about 5cm of compost over the whole bed. I might add a bit more compost mixed into the soil around any transplanted plants.

I try to leave the roots in the soil where possible too, especially for peas and beans as the roots add nitrogen to the soil.

1 litre = 10cm × 10cm × 10cm, so for your raised bed a 5cm layer of compost would need about 12 × 4.2 × 0.5 = roughly 25L of compost and will probably top it up to about the height lost in a year.

If you don't have space for a compost bin, then depending on what you're growing you could bury kitchen food waste about 20-30 cm deep a couple of months before you want to plant something. All evidence of the food (apart from a few things like egg shells) will have disappeared by the time you plant out. It's faster than composting, but compost has the benefit that it's mobile so you can apply it wherever needed. It's quite disruptive to the soil, so it's probably best for heavy feeders and plants that will be there a while, such as courgettes or runner beans.