Prepare the pot. Choose a heavy stovetop-safe pot with a tight-fitting lid that can hold the loose ingredients with headroom. See the pot-size and scaling notes. Brush the inside bottom with the sunflower oil. Do not brown the meat.
Cut the meat. Keep the beef and lamb chilled until ready to assemble. Cut into large, similar 1 1/2–2-inch (4–5 cm) chunks. Keep raw-meat utensils separate from prepared vegetables, and wash hands and clean surfaces after handling.
Prepare the vegetables. Peel the potatoes, carrots, onions, and celeriac. Quarter the potatoes or cut into roughly 2-inch (5 cm) pieces; cut carrots and celeriac into roughly 1 1/4-inch (3 cm) chunks and onions into eighths. Remove the cabbage core and cut the weighed, cleaned cabbage into 2–2 1/2-inch (5–6 cm) pieces. Slice the garlic.
Build the first layers. Spread about one-third of the cabbage and onions over the bottom. Add half of each meat, then half of the potatoes, carrots, and celeriac. Scatter over part of the salt, peppercorns, and garlic rather than seasoning only the top.
Complete the layers. Add another layer of cabbage and onions, then the remaining meat and root vegetables. Distribute the remaining salt, peppercorns, garlic, and bay leaves between the layers. Finish with the reserved cabbage on top. Leave headroom; do not force a lid onto an overfilled pot.
Add the hot water. Pour the measured hot water slowly down the inside edge of the pot. The top vegetable layer should not be completely covered: the cabbage, onions, and meat will release more liquid. Put on the lid.
Bring to a gentle simmer. Heat over medium until the liquid is gently simmering, allowing about 15 minutes. Then immediately reduce the heat to low. Avoid a rolling boil.
Simmer without stirring. Cook, covered, at a quiet simmer for about 2 1/2 hours. Keep the lid closed as much as practical and do not stir the layers. If there are signs the bottom is drying out, check carefully and add a little hot water as needed rather than letting it scorch. Do not add water just to submerge the upper vegetables.
Check safety and tenderness. Begin checking after about 2 hours 15 minutes of gentle simmering. Probe the center of a large piece of each meat with a clean thermometer: whole-cut beef and lamb must reach at least 145°F (62.8°C), followed by at least a 3-minute rest. That safety minimum does not establish tenderness. Both meats should also separate easily with a fork; if firm, continue gently in 15-minute increments, checking the liquid as needed. Potatoes and carrots should be tender throughout.
Rest the stew. Turn off the heat and let the covered pot rest for 15 minutes. Remove the bay leaves. Taste a little broth and adjust seasoning only after cooking, since evaporation can concentrate salt.
Serve gently. Use a ladle to lift meat, vegetables, and broth into bowls without vigorously stirring or breaking up the potatoes. Divide into the listed number of servings, giving each a share of both meats and vegetables. Serve warm with bread if desired.
Cool leftovers promptly. Divide leftovers into shallow containers and refrigerate within 2 hours, or within 1 hour when the surrounding temperature exceeds 90°F (32°C). Do not leave the full deep pot on the counter to cool. Keep refrigerated at 40°F (4°C) or below, use within 2–3 days, and reheat throughout to 165°F (74°C).