These bars are best the day they are made so make sure you have plenty of friends and family around to help you.
For crust: Preheat oven to 375°F. Butter bottom of 13x9x2-inch metal baking pan. Whisk flour, oats, sugar, salt, and cinnamon in large bowl. Add 1 cup butter; rub in with fingertips until mixture sticks together in small clumps. Transfer 2 cups to medium bowl; mix in almonds and reserve for topping. Press remaining crumb mixture evenly onto bottom of prepared pan. Bake crust until golden and just firm to touch, about 22 minutes. Cool 10 minutes.
For filling: Mix all ingredients in medium bowl. Spread evenly over crust in pan, then sprinkle reserved topping over. Bake bars until filling bubbles thickly at edges and topping is golden brown, about 40 minutes; cool in pan on rack. Cut lengthwise into 4 strips, then cut each strip into 6 pieces, forming 24 bars.