Preheat the oven to 350℉. Line a 9×13 baking dish with parchment paper, or spray with nonstick cooking spray.
In a large bowl, stir together the flour, quick oats, light brown sugar, melted butter, pecans, and salt until combined well and crumbly. Remove a heaping 1 cup from the bowl and set aside for later (this will be for the crumble topping).
1½ cups all-purpose flour, 1½ cups quick oats, 1 cup packed light brown sugar, 1 cup (2 sticks) salted butter, 1 cup finely chopped pecans, ½ teaspoon sea salt
Press the remaining mixture firmly into the prepared baking pan to form an even crust.
In a saucepan, over medium heat, add the condensed milk, semi-sweet chocolate chips, and butter. Let it cook, while stirring occasionally, until it's completely smooth and melted.
1 can (14 oz) sweetened condensed milk, 1 cup semi-sweet chocolate chips, 2 tablespoons salted butter
Pour the melted chocolate mixture over the crust in pan. Sprinkle with the reserved crust mixture (set aside from earlier). Sprinkle the mini red and green m&m's over top. Press down lightly with clean hands (this helps the m&m's and crust mixture stay in place).
1 cup Christmas Mini M&M's
Bake for 22-25 minutes. The edges should be lightly golden brown and pulled away from the edges of the pan, but the bars should not look completely set/firm yet.
Remove the pan from the oven and let the bars cool inside the pan, at room temperature, for 1-2 hours before cutting into squares.