1. Cook bacon in a skillet over medium-high heat until crisp; drain on paper towels. Reserve 2 Tbsp. fat in skillet; discard remaining fat. Crumble bacon. Sauté onion in skillet over medium heat for 5 minutes. Add garlic; sauté until onion is tender, about 5 minutes more. Remove from heat.
2. Preheat oven to 375ºF. 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. Press crumb mixture into pan.
3. Sprinkle bacon over crust. Combine cheeses; 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.
4. Bake until tart is golden, 30 to 35 minutes. Let stand for 10 to 15 minutes before serving.