Preheat a grill to medium. Put the peppercorns and coriander seeds in a small plastic bag and crush with a mallet or small skillet. Transfer to a bowl and add the mustard seeds, paprika and dill seeds. Rub the beef with the vegetable oil and season with kosher salt. Rub the pepper mixture all over the beef; set aside.
Make the jam: Cook the bacon in a large nonstick skillet over medium-high heat, stirring occasionally, until browned and crisp, about 7 minutes. Transfer the bacon to a plate, reserving 2 tablespoons drippings in the skillet. Add the onions to the skillet, season with kosher salt and cook, stirring occasionally, until slightly softened, about 5 minutes. Add the beer, vinegar and brown sugar; bring to a simmer. Reduce the heat to low and cook, stirring occasionally, until the onions are very soft and the liquid is absorbed, about 30 minutes. Stir in the bacon and thyme; remove from the heat and set aside.
Meanwhile, brush the grill grates with vegetable oil. Grill the beef, turning occasionally, until well marked, about 15 minutes. Reduce the heat to medium low, cover the grill and continue cooking, turning the beef occasionally, until a thermometer inserted into the center registers 125 degrees F, about 15 more minutes for medium rare. Transfer to a cutting board and let rest, 10 minutes.
Slice the beef, sprinkle with flaky salt and serve with the bacon-onion jam.