Slicing mushrooms on the bias, angling the knife, rather than cutting perpendicular to the cutting board makes for broader but not thicker slices and a professional-looking presentation.
1. Brush or mist trimmed portobellos with oil; season with salt and pepper, if desired. Set aside.
2. Trim ends of 1 orange; place one end on cutting board cut-side down. Angle knife to cut between pith and pulp, and follow curve of orange to remove peel. Cut along inside of membrane walls to remove sections. Repeat with remaining orange. Roughly chop both; place in bowl. Add orange zest, olives, capers, 2 tsp. oil, and shallot to oranges; season with salt and pepper, if desired.
3. Heat grill pan or sauté pan over medium-high heat; brush or rub pan lightly with oil. Cook mushrooms in pan 3 minutes per side; cool.
4. Slice portobellos on bias, fan out across plates, and top with tapenade.