How to Make It
Preheat oven to 325°F. Place bacon in a large Dutch oven over medium. Cook, stirring occasionally, until bacon is browned and crisp, 12 to 13 minutes. Using a slotted spoon, transfer bacon to a plate; pour drippings into a small heat-proof bowl, and reserve.
Toss beef chunks with salt and pepper. Increase heat to medium-high. Add 2 tablespoons of the reserved drippings to Dutch oven; swirl to coat. Add half of the beef in an even layer, and cook until browned on all sides, about 4 minutes per side. Transfer beef to plate with bacon, and repeat with remaining drippings and beef.
Add onions to Dutch oven; cook, turning occasionally, until browned, about 3 minutes. Add carrots, celery, garlic, and thyme; cook, stirring often, until vegetables are lightly browned, about 2 minutes. Add tomato paste; cook, stirring constantly, until vegetables are coated, 1 to 2 minutes. Add flour; cook, stirring constantly, until toasted, about 1 minute. Add Guinness; cook until reduced by half and thickened, about 3 minutes. Add beef broth, cooked beef, and bacon; bring to a boil over high.
Remove from heat. Cover with lid; cook in preheated oven until meat is mostly tender, about 1 hour and 30 minutes. Uncover, stir in potatoes, and return to oven. Cook until beef and vegetables are fork-tender, about 30 to 40 minutes more. Stir in mustard.
Stir together parsley and lemon zest; sprinkle over each serving. Serve hot with crusty bread and butter.
INSTANT POT VERSION:
1. Repeat steps 1 and 2, omitting oven.
2. Add onions and 1 tablespoon reserved drippings to Instant Pot. Cook until browned, about 3 minutes. Add tomato paste and cook, stirring constantly, until onions are coated, about 1 minute. Add flour and cook until evenly coated and toasted, about 1 minute. Add Guinness and cook until thickened and reduced, about 3 minutes. Add 2 cups broth, carrots, celery, garlic, thyme, potatoes, and browned beef and bacon.
3. Cover Instant Pot and turn valve to SEAL. Set Instant Pot to MANUAL. Choose HIGH PRESSURE for 20 minutes (it will take 15 minutes to come to pressure). Quick-release pressure (should take about 5 minutes) and uncover. Stir in mustard.
4. While stew cooks, stir together parsley and lemon zest. Sprinkle over each serving. Serve hot with crusty bread and salted butter.