Put the tomatoes on a sheet pan (covered with either aluminum foil or parchment paper) and spread them out. Put it in the oven and roast for about 45 minutes, until tomatoes are a little charred, bubbly and soft. Make sure to check on them regularly, flipping them over or tossing them around to prevent from sticking to the foil or paper. Bring a pot of salted water to boil. (It may take longer than 45 minutes depending on what type of oven you have.)
Cook your pasta, reserve about a cup of the salted pasta aside. DO NOT RINSE, only drain it (rinsing pasta takes the starch out, therefore making the pasta unable to stick to or absorb any sauce afterward). In a medium size pot, add olive oil, garlic, red chili pepper flakes and onions. Cook on medium heat for about 5 minutes, then add the roasted tomatoes. Cook for 2-3 minutes (until onions are soft), turn off the heat and use a hand blender or blender to puree the mixture into a smooth sauce.
Add your butter, the tomato sauce, stir, then add pasta water a little at a time until desired consistency. I use a little sugar to adjust the sweetness of the sauce (never more than a teaspoon) and some salt to balance it out.