In a large, heavy-bottomed pot, melt butter and oil over medium-high heat until foaming subsides. Add onions and salt and cook until onions begin to soften and release their liquid, about 5 minutes. Lower heat and cook, stirring often, until onions are soft and just beginning to brown, about 15 minutes. Add diced pears and continue cooking, stirring often, until mixture is golden, about 40 minutes.
Stir in brown sugar, mustard, vinegar, water, thyme, and pepper. Bring to a bubble, lower heat, and simmer until pears are soft and jam is thick and glossy, about 15-20 minutes. If desired, use a potato masher to break up fruit to reach your preferred consistency (I like to leave my jam chunky). Jam is best served slightly warm or at room temperature.
Store the jam in the refrigerator in an airtight container for 2 weeks. The jam can also be frozen, leaving about 1/2-inch at the top of your containers for expansion.