How do I cook a 15lb pork sirloin roast (bone-in)?

Brine it, season it, smoke it.

I would not think a bone in pork sirloin roast of that size will pose a problem with being dried out. Usually that cut comes with a nice fat cap covering, and plenty of fat marbling in the meat itself. You could soak it in a simple salt brine if you wished, but I've never found that necessary myself. I prefer to cook large pork roasts with the low and slow method, at about 300F rather than the usual 350F or higher. My pet method for a large piece like you have is to season it the day before, using an herb paste I make by grinding up a the cloves from a head of garlic with 1-2 teaspoons of kosher salt and a teaspoon of pepper, plus a good size pinch of rubbed sage. I make small punctures deep into the meat all over, and smoosh some of that paste inside the holes. Then I will rub whatever is left all over the outside of the roast itself. If there isn't much left, then I will use a mix of salt, pepper, and garlic powder on the outside. Let that sit covered in the fridge overnight. Next day, take it out about an hour or so before you start the roasting, to let it warm up. I usually allow about 35-40 minutes per pound of roasting time, and in the case of your roast, that means roasting for about 8 1/2 - 9 hours. When the internal temp hits about 150 on an instant read thermometer, take it out and let it rest at least half an hour. If you want really crispy fat on top, toward the end of the resting time, heat up the broiler on your oven. Return the roast to the oven and broil it until that fat cap is toasty delicious and golden brown. Remove and carve. I'm not a big fan of sauces, because I think well cooked meat will speak for itself. However, pork loves apples, and spiced fried apple slices would never go astray. You could also consider a honey mustard sauce, or a spiced cherry sauce. You just want a sauce a bit on the tangy side, since pork is a rich and fatty meat it needs that acidity for balance. Honestly, I'd suggest keeping things simple. As I said, good meat well roasted should be allowed to shine for itself. Some lilies don't need gilt.