Cook bacon in a skillet over medium-high heat until crisp; drain on paper towels. Reserve 2 tbsp. bacon drippings in skillet; discard remaining drippings. Crumble bacon. Saute onion in skillet over medium heat 5 minutes more. Remove from heat.
Preheat oven to 375 degrees. Line a baking sheet with foil. Gently mix cracker crumbs, butter and 1/2 cup hot water. Mist a 9-inch tart pan with cooking spray; place on baking sheet; place on baking sheet. Press crumb mixture into pan.
sprinkle bacon over crust. Combine chesses; spread 1/2 cup over bacon. Spread onion mixture and broccoli on top. Sprinkle with remaining cheese. Whisk milk, mustard, flour and thyme; pour on top.
Bake until tart is golden, 30 to 35 mintues. Let stand 10 to 15 minutes before serving.