Heat 3/4 cup vegetable oil in a large skillet over medium heat; cook and stir onions until golden brown and very tender, 20 to 30 minutes. Add garlic and butter; cook and stir until butter is melted and garlic is fragrant, 5 to 10 minutes. Remove skillet from heat and cool onion mixture to room temperature.
Combine warm water and olive oil in the bowl of a stand mixer.
Combine bread flour, white sugar, yeast, and kosher salt in a bowl; sift into the bowl with oil and water. Attach dough hook to mixer; add parsley, rosemary, and marjoram and mix on medium speed until dough holds together and is no longer sticky, 5 to 8 minutes.
Turn dough onto a floured work surface and knead, adding flour if needed, until dough becomes somewhat firm. Divide dough into six 10-ounce portions.
Roll each portion of dough into a 12-inch rectangle, dusting with flour if sticky. Fill each rectangle with equal quantities onion mixture and 1/4 cup blue cheese. Roll each rectangle into an oval baguette-shape stromboli, pinching the edges to close.
Grease 2 baking sheets with 1 tablespoon vegetable oil.
Generously brush the bottoms of each stromboli with water and transfer to the prepared baking sheets; cover with plastic wrap. Allow stromboli to rise for 2 hours.
Preheat oven to 375 degrees F (190 degrees C).
Bake in the preheated oven until golden brown, rotating the stromboli as needed for even browning, 45 minutes to 1 hour.