Toast the walnuts on a baking sheet in the preheated oven for 8 minutes. Remove from the oven and cool to room temperature.
Heat the maple syrup in a 1 1/2-quart saucepan over medium-high heat. When the syrup begins to boil, reduce the heat to medium and allow the syrup to continue to boil and thicken for 10 minutes, stirring occasionally with a metal spoon. Remove the very hot syrup from the heat. Immediately add the walnuts to the syrup and stir to combine. Transfer the glazed walnuts to a baking sheet with sides. Use a metal spoon to spread the walnuts evenly over one half of the baking sheet. Set aside.
Place the sugar and lemon juice in a 3-quart saucepan. Stir with a whisk to combine (the sugar will resemble moist sand). Caramelize the sugar by heating for 10 to 10 1/2 minutes over medium-high heat, stirring constantly with a wire whisk to break up any lumps (the sugar will first turn clear as it liquefies, then light brown as it caramelizes. Remove the saucepan from the heat, add the unsweetened chocolate and stir to dissolve. Immediately and carefully pour the caramelized mixture over the walnuts, covering all the nuts. Allow to harden at room temperature for at least 30 minutes.
Invert the praline onto a clean, dry cutting board (it should pop right out of the baking sheet), this will keep the shiny side looking shiny after cutting. Use a sharp serrated slicer to cut (use a sawing motion) the praline into desired size pieces. Store the pralines in a tightly sealed plastic container until ready to devour.