Set large pot filled with heavily salted water over high heat. Clean and dice parsnips*. Once boiling, add in parsnips and boil for 20 minutes or until tender. Drain and set back into same pot to dry slightly.
Preheat oven to 425 deg F. As oven heats, pour beer into small saucepan and heat on high to reduce slightly, 5 minutes. Once reduced, combine with honey, mustard, juice of the lemon, dill, and salt and pepper, set half aside for sauce for finished dish. Glaze onto salmon and bake on sheetpan for 10-12 minutes.
As salmon bakes and parsnips boil, dice and roast cauliflower on sheetpan in oven for 8 minutes. Once cooked, add on minced garlic and cook for another 2 minutes to soften.
Combine cauliflower, garlic, drained parsnips, butter, and cheese into large bowl. Whip using hand mixer for 2-3 minutes or until smooth. Serve with roasted salmon and reserved Guinness sauce.
*I didn't bother peeling the parsnips because they were quite small. If they are large or woody, peel like they are a carrot.