Indoor seed starting. I start tons of stuff from seed, indoors. If you have a good enough seed starting set up, you can start pretty early and just keep up-potting, so by the time stuff can go in the ground, it is pretty good sized.
16 running feet of lights!
And then yes, all the season extenders, hoop house, greenhouse, cold frame, cloches/ hot caps,
Here's someone gardening year round in Minnesota in what they call deep winter greenhouse. They aren't as specific as they should be, but I'm thinking it is partly underground.
https://www.mprnews.org/story/2014/02/03 ... reen-house
Here's a pit greenhouse, that is all underground, except the roof:
https://www.treehugger.com/green-archite ... round.html
Here's some people doing geothermally heated greenhouse with ground source heat pump.
https://www.cleanenergyresourceteams.org ... -altura-mn
But short of that, you can do hoop houses and poly tunnels quite easily. I collect the metal frames that political yard signs come on. They are a good size for a low tunnel. Just push them in to the ground, cover with plastic and secure around the edges.