Pat the beef cubes dry with paper towels, then season them evenly with the salt and black pepper.
Heat 2 tablespoons of the olive oil in a large Dutch oven over medium high heat until shimmering.
Sear the beef in batches in a single layer, turning occasionally, until deeply browned on most sides, then transfer the browned beef to a plate and repeat with the remaining meat, adding more oil if needed.
Reduce the heat to medium and add the butter to the pot, allowing it to melt and pick up the browned bits from the bottom.
Add the onion and celery to the pot and cook, stirring often, until softened and lightly golden, about 5 to 7 minutes.
Stir in the diced red bell pepper and cook until it begins to soften, about 3 minutes.
Add the minced garlic, paprika, cumin, cayenne, oregano, and thyme, stirring constantly until fragrant, about 1 minute, being careful not to burn the spices.
Stir in the tomato paste and cook, stirring, until it darkens slightly in color and coats the vegetables, about 1 to 2 minutes.
Pour in the beef broth and water, stirring and scraping the bottom of the pot to release any browned bits into the liquid.
Add the Worcestershire sauce and bay leaves, then return the seared beef and any accumulated juices to the pot and stir to combine.
Bring the stew just to a gentle boil over medium high heat, then immediately reduce the heat to low so the liquid is at a steady simmer.
Cover the pot with a lid slightly askew and simmer, stirring occasionally, until the beef is very tender, about 75 to 90 minutes.
Stir in the zucchini and cauliflower florets, then cover again and simmer until the vegetables are tender but not mushy, about 15 to 20 minutes.
Remove the bay leaves and discard them, then stir in the chopped parsley and lemon juice if using.
Taste the stew and adjust the seasoning with additional salt, black pepper, or cayenne to reach your preferred level of saltiness and heat.
If the stew is thicker than you like, stir in a splash of hot water or broth, or if it is too thin, simmer uncovered for a few extra minutes to reduce slightly.
Ladle the spicy keto beef stew into bowls, garnish with extra parsley, and serve hot on its own or with cauliflower rice or keto bread for soaking up the broth.