Heat olive oil in skillet over medium heat. When oil is hot, add onions and garlic and stir frequently until onions are tender and garlic is fragrant. Remove with a slotted spoon (this is why you need to keep the onion and garlic pieces big), shake off excess oil (you don’t have to go crazy here, a little olive oil never killed anyone!), and transfer to your blender. Increase heat to medium-high.
In a large Ziploc bag, combine flour, 1 tsp. salt, and 1 tsp. black pepper. Add chicken pieces, seal bag, and shake to coat pieces with flour. Dump the entire bag into the hot pan and stir quickly to prevent pieces from sticking together. Saute until chicken is golden. Remove chicken with slotted spoon and drain on a paper towel. Turn off the heat under the oil.
While chicken is draining, add wine, chicken broth, tomato paste, thyme, and marjoram to the onions and garlic in the blender. Blend until smooth. Place chicken and bay leaf in your slow cooker and then pour the sauce over the chicken. Place lid on slow cooker and cook on high for 3.5-4 hours. When you have about 1/2 hour to go, add the mushrooms and the chopped green pepper. Cook until mushrooms and pepper are tender. Remove bay leaf and serve over pasta.