Preheat oven to 400 degrees. Lay the pepperoni slices into a single layer on the baking sheet. Cook for 5-10 minutes on one side, flip and cook on other side for about 3 more minutes.
Let the pepperoni cool and then dice into large chunks once they are cool. Reserve the pepperoni oil on the sheetpan.
While the pepperoni slices are cooking, heat the olive oil in a large pot (or Dutch oven) over medium-high heat. Add the onions and carrots and cook until soft, about 5 minutes. Add the garlic and cook for 30 seconds.
Add the crushed tomatoes, tomato sauce, sugar, herbs, salt, pepper and crushed red pepper flakes. Bring to a boil and then lower heat to low and let simmer for 25 minutes.
After the sauce has had a chance to simmer, turn the heat off and CAREFULLY use an immersion blender to smooth out the sauce. Keep the immersion blender down in the marinara or you risk spraying yourself with hot marinara. You can leave it chunky if you like. Or you can CAREFULLY put the sauce into a blender (in 2 batches).
Once the marinara is smooth, turn the heat back on low and add the chopped pepperoni and the reserved pepperoni oil. Cook for 10-15 more minutes.
Serve the marinara over any cooked pasta. Can store in the refrigerator for up to one week. Or let the sauce cool and it freezes really well for up to 3 months.