Pat beef dry and season with salt and pepper. Heat oil in a Dutch oven over medium-high heat and brown beef in 2 batches for 8 to 10 minutes total; transfer to a plate.
Cook onion in the same pot for 5 minutes. Add garlic and tomato paste for 1 minute, then sprinkle in flour and stir for 1 minute.
Gradually add stout and broth, scraping the browned bits from the bottom. Return beef and add thyme, then cover and simmer gently for 1 1/2 hours.
Add potatoes, carrots and parsnips. Simmer partly covered for 35 to 45 minutes until vegetables are tender and beef reaches at least 145°F and yields easily to a fork.
Freeze after cooking: divide stew among 6 shallow containers, refrigerate within 2 hours and chill fully. Leave 1 inch of headspace, seal, label and freeze for up to 3 months.
Thaw overnight in the refrigerator. Reheat the stew in a covered saucepan over medium-low heat for 15 to 20 minutes, stirring gently and adding reserved water as needed, until every portion reaches 165°F.
Serve one-sixth of the beef, vegetables and gravy per portion.