Stuffed Potatoes with Lamb & Spinach

• 3large russet potatoes
• 3 tbspolive oil
• 1 tbspbutter — melted
• ½red onion — chopped
• 1 lbground lamb — or ground beef
• 1 tspdried oregano
• 1 tspdried rosemary
• 1 tspdried thyme
• 1 tspkosher salt
• ½ tspred pepper flakes
• 2 cupsspinach — packed
• ½ cuppecorino romano — grated
• 1 cupwhite cheddar — shredded

Tzatziki Sauce

• 1 cupgreek yogurt
• ½ cupcucumber — medium-sized, grated, ½
• 1 tbspdill — chopped
• 1 tbspmint — chopped
• 1garlic clove — pressed or minced
• ½lemon — juice, zest
• 1 tbspolive oil
• —Salt — to taste