In a small bowl, combine the flour, oats, baking powder, cinnamon, and salt. Whisk to combine.
In a stand mixer with the paddle attachment (or in a large bowl with a hand mixer) beat the butter with the brown and granulated sugars until light and fluffy at medium speed. Add the eggs one at a time until each is beaten into the mixture. Add the vanilla extract and beat until combined.
Reduce speed to low and slowly add the flour mixture a little at a time until incorporated. Add the raisins, coconut, and walnuts and stir until just combined.
Scrape the dough off the paddle and put the bowl in the refrigerator to chill 15-30 minutes (see note).
While the dough is chilling, preheat oven to 350F.
Using a 1.5 inch scoop, place balls of the dough on a silicone mat (or parchment) lined baking sheet. Leave them a couple inches apart. Gently press down on them so the tops are slightly flattened but the dough is still tall. Place the remaining dough back in the refrigerator to keep it cool.
Bake for 12-13 minutes until the edges at the bottom are just starting to brown. Remove from the oven and immediately sprinkle with the sea salt. Transfer to a cooling rack to finish cooling.
Repeat with the remaining dough until all cookies are baked.
The colder your dough, the less they will spread. Cold dough that is pretty firm when chilled will result in smaller, round, mounded cookies. Slightly cool dough will spread out more as in the picture. Warm dough could result in the butter melting out of the cookie and is not recommended.I got 36 cookies when using a 1.5 inch scoop.