Place flour, salt, and yeast in the bowl of a food processor and pulse to combine. With the processor running, slowly add 2 cups water heated to 115 degrees until a dough forms. Add 2 tbsp. oil and pulse until just combined. Transfer dough to a lightly floured work surface and knead briefly until smooth, about 1 minute. Lightly grease a bowl with 1 tbsp. oil; place dough in bowl and cover with plastic wrap; let rise until doubled in size, about 1 hour.
Heat oven to 450°. Lightly grease a 13" x 18" rimmed baking sheet with 2 tbsp. oil and transfer dough to baking sheet. Press and stretch the dough to cover the bottom of the pan. Cover loosely with plastic wrap and let rise for 30 minutes. Remove plastic wrap and, using your fingers, lightly dimple the dough. Drizzle more oil over dough and sprinkle with sea salt; bake until light golden brown, about 30 minutes. Remove focaccia from oven and, while still warm, spread evenly with 'nduja, if using. Cut into squares and serve warm or at room temperature.