Before starting, preheat the oven to 450°F.Wash and dry all produce. Remove 1 tbsp butter (dbl for 4 ppl) from the fridge and set aside to come up to room temperature. Cut potatoes into 1/2-inch wedges. Add potatoes and 1 tbsp oil to a baking sheet. Season with salt and half the Montreal Steak Spice, then toss to coat. (NOTE: For4 ppl, use 2 baking sheets, with 1 tbsp oil per sheet.) Roast in the middle of the oven, flipping halfway through, until golden-brown,25-28 min. (NOTE: For 4 ppl, roast in the middle and the top of the oven, rotating sheets halfway through.)

While potatoes roast, thinly slice cucumber. Add vinegar, 1/2 tbsp sugar and a pinch of salt (dbl both for 4 ppl) to a medium microwaveable bowl. Microwave, in 15second increments, stirring occasionally, until sugar dissolves. (NOTE: This is your pickling liquid.) Add cucumbers to the bowl, then stirto combine.

Stir together mayo and horseradish in a small bowl. Season with salt. (NOTE: This is your horseradish aioli.) Whisk together 1/2 tbsp pickling liquid (from step 2) and1 tbsp oil (dbl both for 4 ppl) in a large bowl. Season with salt and pepper. (NOTE: This is your vinaigrette.)

Heat a large non-stick pan over medium-high heat. While the pan heats, pat barramundi dry with paper towels. Sprinkle remaining Montreal Steak Spice over the tops of barramundi only, avoiding the skin-side. Sprinkle flour over entire fillets. When the pan is hot, add 1/2 tbsp oil (dbl for 4 ppl), then barramundi, skin-side down. Cook until skin is crispy, 4-5 min. Flip and cook until barramundi is opaque and cooked through, 2-3 min. Carefully remove skin from barramundi, if desired.

While barramundi cooks, halve buns. Add buns directly to the top rack of the oven, cut-side up. Toast until golden-brown,3-4 min. (TIP: Keep an eye on them so they don't burn!)

Add spring mix to the bowl with vinaigrette. Toss to combine. Drain cucumbers, discarding pickling liquid. Spread horseradish aioli on bottom buns. Spread1 tbsp butter (dbl for 4 ppl) on top buns. Place barramundi on bottom buns, then top with pickled cucumbers, dressed greensand top buns. Serve sammies with potato wedges alongside.