Butter a 13x9-inch baking pan and set aside.
Combine the oats, sugar, and salt in a food processor and pulse eight to ten times to chop the oats well. Add the flour and pulse several times to mix. Scatter the butter pieces over the dry mixture. Pulse until the mixture resembles a fine meal. Sprinkle the water over the mixture and pulse again, just until the ingredients start to form coarse, clumpy crumbs. (The crumbs should pack easily when pressed between your fingers.) Spread the mixture evenly in the prepared pan, pressing it with your fingertips to form a level layer on the bottom with a lip extending about ¼ inch up the sides of the pan. Refrigerate the crust for 15 minutes.
Preheat the oven to 350°F. Bake the crust on the center oven rack for 20 minutes. Cool on a rack.
While the crust cools, make the filling. Combine the maple syrup, brown sugar, cream, and butter in a medium saucepan. Bring to a full boil and boil for 30 seconds. Remove the pan from the heat and stir in the vanilla, salt, and pecans. Pour the filling over the cooled crust, spreading it evenly with a spoon. Bake on the center oven rack for 15 minutes. The filling should boil as it bakes. Transfer the pan to a rack and cool completely. Cover and refrigerate for a couple of hours, or overnight, before slicing. Slice cold, in small portions (these are sweet and rich), and serve at room temperature.
Buy maple syrup by the gallon; it's more economical. Refrigerate or freeze syrup for long-term storage. Use 1-quart mason jars and leave at least 1 inch of headspace, if freezing. Liquefy crystallized maple syrup by placing the container in a pan of very hot water.