In a large bowl add flour, unsalted butter, salt and instant yeast. If using regular yeast, bloom it with salt, sugar and warm water before using in the recipe.
Add warm water and mix well using a stand mixer or with your hands. Bring everything together and make a soft and smooth dough.
Cover the dough ball with some oil or butter and place in the instant pot. Set the mode to yogurt (low) and time to 60 minutes.
Once the dough rises and doubles, remove it.
Meanwhile, melt the butter.
Add garlic powder, chili flakes and oregano to it and mix well.
Divide the dough into 2 parts and set one half aside.
Using the other half on a baking mat; roll out into a thin rectangle of about 9*13 inches.
Spread marinara sauce on the dough, but leave ½ inch on the edges to prevent the sauce from oozing out.
Place onions and peppers in a thin layer.
Arrange the mushrooms on top.
Next, add the spinach leaves in a thin layer. Try not to add too much as it might not roll well.
Spread the mozzarella and cheddar cheese on top.
Starting from one long side, start rolling the dough into a tight roll; making sure the fillings do not slide away.
Pinch and seal the edges and ends.
Make a few cuts on top. Preheat the oven to 400 F.
Brush the seasoned butter on it and place in the preheated oven.
Bake for 22-25 minutes and serve sliced. This Stromboli tastes best when served with marinara sauce on the side. Repeat the steps for the remaining half of the dough.