Collect edible plants from nature like nettles and dandelions. There is an app called Vild Mad, that can help you identify wild plants and learn how to use them. For example, you can use nettles to replace spinach and kale.
If you have space, get a few chickens. You can start with 2-3 chickens (one chicken costs 30 DKK).
Ask the butcher if he has cheaper meat cuts in the back.
Do not buy vegetables or fruits per item. Instead look at the per kilo price. The rule of thumb is that two out of three vegetables in the basket must cost less than 20 kroner per kilo.