Saute bacon over medium-high heat in large pot for 10-11 minutes until crisp. Drain on paper towels. Pour bacon fat into bowl.
Season beef with salt and pepper.
Return two tablespoons bacon fat to pot and heat to medium-high. Brown beef in batches 8-10 minutes, transfer to bowl.
Reduce heat to medium; warm 1 tablespoons bacon fat. Cook onions, carrots and celery for 10 minutes. Add garlic and tomato paste; cook 1 minute. Add mushrooms, saute 10-12 minutes. Stir in demi-glace, mustard, beef, bacon, broth, salt and pepper. Bring to simmer; transfer all to slow cooker. Cover; cook on high for 4 hours. Skim off fat. Stir in parsley.