Peel potatoes, sweet potato and parsnip and cut into chunks. Place in a medium saucepan and cover with water by 1 inch. Add garlic, if using. Salt generously. Bring to a boil over high heat with pan partially covered. Reduce heat to medium and cook partially covered until vegetables are just tender when pierced with a knife point, about 15 minutes. The parsnip may take longer to become tender, so make sure you test it as well.
Drain potato mixture and return to pan, placing pan on still warm burner to steam off excess moisture. Add butter and milk and mash with hand masher until fairly well combined. Adjust seasoning with salt as needed. Can be made ahead up to this point and kept covered on stove, up to 1/2 hour. Gently reheat over very low heat before serving.
Kitchen Notes
Picking the perfect parsnip. Parsnips look like pale yellow-white versions of their cousins, carrots. You want them to have the same firmness as carrots. Choose parsnips that aren’t shriveled or spotty. Also, go for smaller parsnips when you shop—larger ones can tend to be woody.You probably want the garlic. Adding garlic will give a nice savory note to this mash-up, tempering its sweetness. I highly recommend including it. That said, if you’re serving this as a side with a non-garlicky main course, don’t skip the garlic.