Buttermilk Blue Potatoes Au Gratin

• 2 shallots
• 1 1/2 potato chips
• 1 pimento (canned)
• 1 garlic clove
• 1 monterey jack
• 350 ml milk
• 260 gr mushrooms (raw)
• 240 ml buttermilk
• 110 gr pkg broccoli (frozen chopped)
• 60 ml mayonnaise