Preheat oven to 350°F.
Heat oil and butter in a large heavy casserole or Dutch oven over medium heat.
Add onions and garlic and cook, stirring, until the onions are soft and beginning to brown, 8 to 10 minutes.
Stir in tomato paste, paprika and marjoram.
Season beef with 1 teaspoon Dirty Croc. Add the beef and portobelo mushrooms to the pot; gently stir to combine. Add broth and cover the pot with a tight-fitting lid.
Transfer the pot to the oven and bake until the beef is very tender, 1 3/4 to 2 1/2 hours. Stir in mushrooms caps and continue baking, covered, for 15 minutes more. Remove from the oven, uncover and let stand, undisturbed, for about 15 minutes.
Skim or blot any visible fat from the stew. Transfer the beef and mushrooms to a bowl with a slotted spoon. Return the pot to the stove and bring to a gentle simmer.
Cook until the sauce just coats a spoon. Stir the beef, mushrooms and the remaining 1/2 teaspoon Dirty Croc Seasoning into the sauce and heat through, about 1 minute. Pepper to taste Serve garnished with tarragon (or dill), if desired.