In a large pot, melt the stick of butter over medium-high heat. Add the sweet onions, red onions, leeks, green onions, salt and pepper. Lower the heat to medium and sauté the vegetables until soft, about 25-30 minutes. Meanwhile, in a small saucepan, melt the 4 tablespoons of butter over medium-high heat and whisk in the flour. Brown the roux until it has deepened in color and smells nutty, whisking often. To the soup pot, add the garlic, Dijon mustard, Worcestershire sauce, thyme, and bay leaves. Cook another minute or two. Slowly add 1 cup of the chicken stock to the roux and whisk to incorporate. Add the roux mixture plus the remaining 7 cups of chicken stock to the soup pot. Bring to a boil and then reduce to a simmer. Simmer the soup for 20 minutes before stirring in the heavy cream and remaining salt and pepper. Serve warm.
Slice the pretzel rolls into thin slices, about ⅜ of an inch each. In a large sauté pan, heat the oil over medium-high heat. When the oil is hot, add the pretzel roll slices in an even layer. Do not overcrowd the pan (you might have to work in batches). Cook the croutons for just a minute or two on each side until golden brown. Transfer the croutons to a paper towels and immediately sprinkle each one with the shredded Parmesan cheese.
Serve the soup with a couple of pretzel roll croutons on top. Garnish with the chives.
Leeks are full of dirt and must be cleaned well. I first slice off the roots as well as the darker green ends. Then, I slice the leeks into circles and add them to a colander, making sure to separate each slice into its layers since that is where most of the dirt will be. I run them under colder water and use my hands to agitate them in order to get all of the dirt out. Once dried, they are ready for the soup.