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