I was given a large bag of very dark roast and it tastes like ash — can I brew around it?

The specific adjustments, and they compound:

Lower the water temperature. This is the single biggest one. Well off the boil — let the kettle stand for a minute or two. In a press, adding a little cool water to the grounds before the hot water works too. Bitter compounds extract preferentially at high temperature.

Grind coarser. Less surface area, slower extraction. Go noticeably coarser than you would for the same method with a medium roast.

Shorten the contact time. In a press, four minutes becomes two and a half. Plunge and decant immediately rather than leaving it sitting.

Use more coffee, less time. A higher dose with a shorter, gentler extraction gives you strength without pulling the harsh compounds. This is counter-intuitive and it works.

For the stovetop pot specifically: start with warm water in the base, take it off the heat as soon as it flows, and run it on a low flame throughout. Most stovetop bitterness is the last spluttering phase, and stopping before it happens fixes a great deal.

Some of it is unavoidable and a lot of it is not, so it is worth being clear about which is which.

Unavoidable: at very dark roast levels, some of the material has genuinely carbonised. Those compounds are in the bean and they will end up in the cup. You cannot brew out something that is physically present.

Avoidable: most of what people experience as harsh bitterness in dark roasts is over-extraction. Dark roasted beans are more porous and more brittle than lighter ones, so they give up their solubles much faster. Brewing parameters tuned for a medium roast will over-extract a very dark one badly, and over-extraction is where the acrid, drying, ashy character comes from.

So the strategy is to extract less, deliberately, at every lever you have.

The warm-water start in a stovetop pot deserves emphasis because it changes the drink more than any other single adjustment.

Filling the base with already-hot water means the pot reaches brewing pressure quickly, so the grounds spend far less time sitting in a heating chamber slowly cooking. Cold start, and the coffee bed is baked for several minutes before anything flows.

Use a cloth to handle it, since the base gets hot fast, and take it off the heat the moment the flow turns pale and starts to sputter. Run it under a cold tap for a second to stop the extraction dead.

Between that and a coarser grind, a dark roast in a stovetop pot goes from harsh to acceptable in one attempt.

If it still is not enjoyable after all that, the honest options are to use it differently rather than to keep fighting it.

Very dark roasts work far better with milk — the fats and sugars round off exactly the harshness you are describing, which is why this roast level is popular for milk drinks. They are also good for cold brew, where the long low-temperature extraction leaves most of the bitter compounds behind, and a large bag is exactly the right quantity for that.

A kilogram is a lot of coffee to dislike. Cold brew would get through it pleasantly.