My kitchen freestyle recipes aren't as formal as other recipes on this site, as they basically tell the story of how I threw a dish together, as a way to bring you into the kitchen with me. In the case of this recipe, I don't think you need me to tell you how to mash potatoes. But I will say that I mashed my potatoes with olive oil instead of butter, to complement the Mediterranean flavours of the sautéed chard. I didn't watch the clock, but I probably pulled this dish together in the time that the sausages were roasting in the oven, about 45 minutes, including several pauses to take photos (which I'm guessing you won't do).
While the potatoes are cooking, prepare the Swiss chard. Holding the bunch together, slice the stems into about 1/2-inch lengths, right up to the base of the leaves. Wash the stems and set aside. Wash the leaves (I found a salad spinner helpful for this) and then roughly slice them crosswise into about 1-inch lengths. If you find other long pieces of stems, slice them and add to the rest.
Add the olive oil to a sauté pan over medium high heat. When the oil has warmed, add the diced red onion, garlic and Swiss chard stems. Cook for a few minutes, stirring occasionally, taking care that the garlic doesn't brown (turn the heat down if you need to). Season with a moderate amount of salt and pepper (you'll be tasting and adjusting as the cooking progresses).
You can either steam the Swiss chard leaves in a lidded steamer basket set over the cooking potatoes concurrently with the step above, or add the washed leaves to the sauté pan. Either way, turn the leaves occasionally with tongs until they cook down and soften. Then either transfer the steamed leaves to the sauté pan, or continue to sauté them, adjusting the heat as required and adding more olive oil if the chard seems too dry. Make sure you mix the leaves well with the stems and aromatics, and cook until tender but still characterful (by which I mean to remind you that vegetables should have texture and not be mushy). Taste and add more salt and pepper as needed.
If you're going to serve the sautéed chard on its own as a side dish, transfer to a serving bowl and optionally sprinkle a bit of salt and pepper over to finish. But since we're making a Swiss chard mash here, drain and mash the potatoes to your liking while the chard is finishing up. Then move everything out of the sauté pan and onto the cutting board for another quick chop. Run the knife through at 1/2-inch intervals first in one direction and then the other. Finally, add the chard mixture into the mashed potatoes, stir it in well and give it a final quick mash.
Decant the mash to a serving bowl and enjoy. I reheated leftovers the next day in a small non-stick pan with olive oil, creating the added benefit of some crusty goodness, proving once again that leftovers rule.