Preheat the oven to 450 degrees. Toss the sliced mushrooms with the avocado oil and the seasoning on a sheet pan until well-combined. Roast for 20-25 minutes, until golden brown.
Mix all the sauce ingredients in a bowl until well-combined. Taste and season.
Toast the bread for the last few minutes on the pan with the mushrooms. Split the bread, spread with pesto rosso, roasty mushrooms, havarti (pop it in the oven again for a minute if you want it to melt), arugula, and the top piece of bread shmeared with the horseradish sauce. OH MY WORD can you even? This is so good.
