In a large mixing bowl or the bowl of the stand mixer, combine all the bread flour, 1 cup of whole wheat flour, yeast, salt, sugar, oil and milk. Mix well to form a smooth, pliable dough. Add more wholewheat flour if the dough seems too wet.
Place the dough in a lightly greased bowl, cover and set aside to rise for 60~90 minutes.
While the dough is rising, make the filling. Heat 1tbsp oil in a pan, add the onions, garlic and cook till the onions turn browned around the edges.
Add the chopped spinach and cook till it's wilted. Season with salt and pepper. Turn off the heat and let the filling cool,
Gently deflate the dough and roll it out into 12"x18" rectangle (for bigger rolls) or divide the dough in half and roll each piece into 8"x10" rectangle.
Spread the dough evenly with the marinara sauce, cheese and spinach filling.
Roll the dough, starting with the long end, into a log. Cut the log into 12~18 equal pieces. Place the rolls on a parchment lined baking sheet. If you want to make heart shaped rolls, then press the roll lightly on the top and bottom to make the heart shapes.
Cover lightly with a plastic wrap and let rise for 60~90 minutes.
Preheat the oven to 350°F. Bake for 25~30 minutes or until the rolls are lightly golden and the cheese has melted.
Serve right away with some marinara sauce on the side.