Place the diced rhubarb, lemon juice, sugar and lemon zest in a small bowl and stir to combine. Set aside.
Preheat oven to 375°F and line 2 baking sheets with parchment or silicone mats.
In a small bowl, combine flour, baking soda, and salt; set aside.
In a large bowl, cream butter and sugars until light and creamy with a hand mixer or the flat beater of a stand mixer. Beat in egg, vanilla and lemon oil or extract until throughly combined. Add the flour mixture slowly until combined but don't overmix. Stir in lemon zest and rhubarb mixture (draining the mixture a bit if it's too wet).
Drop tablespoon-sized rounds of dough about 2 inches apart onto the prepared cookie sheets. (A small cookie scoop works great here.) Don't flatten the dough rounds. Bake 10–12 minutes or until the cookies are slightly golden around the edges, switching and rotating the baking sheets if necessary for even baking. Cool on the cookie sheets for a few minutes to set, then transfer to a wire rack to cool completely. Store in airtight container.
