Season the tri tip liberally with Red Tuxedo seasoning. Add a MeaterPlus probe to the roast so you can monitor the cook time and doneness from the Meater App. Set the tri tip at room temperature while you heat the grill.
Heat a grill for two-zone cooking with the indirect side set around 375 degrees F and the direct side for high heat.
Throw the tri tip on the grill and set your MeaterPlus probe to your desired temp — ours is 130 degrees F for tri tip. Sear the tri tip over direct heat for 2 to 3 minutes then move to indirect heat until it reaches your desired temperature, about 22 minutes.
While the tri tip roast, prep the rest of the sandwich. Prepare the horseradish sauce by combining the mayo, sour cream, buttermilk, prepared horseradish, Worcestershire sauce, salt and pepper in a medium mixing bowl and whisk to combine. Chill in the fridge for at least 30 minutes.
Remove the tri tip from the grill to rest — the MeaterPlus will let you know when the roast is perfectly rested. While the tri tip rests, slice open the bread, drizzle it will olive oil and toast it over direct heat for 2 minutes. Flip the the bread pieces over and lay on the Havarti, close the grill to help the cheese melt. Remove from the grill.
Slice the tri tip into its two muscle sections and then thinly slice each section against its grain. Stack all of the slice tri tip onto one half of the bread, season with additional kosher salt and then top with the arugula and onions. Spread some of the horseradish cream sauce onto the other piece of bread and top to complete the sandwich. Slice the sandwich into wedges and serve.