In a skillet, heat oil over medium heat for 30 seconds. Add onion and cook, stirring, until softened, about 3 minutes. Add garlic, Italian seasoning, salt and peppercorns and cook, stirring, for 1 minute. Add tomatoes with juice and bring to a boil. Remove from heat and set aside.
In a bowl, combine eggs and mustard. Beat to blend. Add milk and beat well. Set aside.
Lightly grease the slow cooker stoneware. Spoon in one-third of the tomato mixture. Spread half the bread evenly over top and sprinkle evenly with half the cheese. Repeat, finishing with final third of tomato mixture. Pour milk mixture evenly over top.
Place two clean tea towels, each folded in half (so you will have 4 layers), over top of stoneware to absorb the moisture. (This is to prevent accumulated moisture from dripping on the bread by absorbing the liquid generated during cooking).
Cover and cook on low for 6 hours or high for 3 hours, until pudding is set and edges are browning.