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1
Bring orange juice, chipotles, and vinegar to a boil in a 1- to 1 1/2-quart heavy saucepan, then reduce heat and simmer, stirring occasionally, until reduced to a generous 1/3 cup, 15 to 20 minutes.
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2
Remove from heat and cool.
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3
While sauce cools, prepare charcoal or gas grill for cooking.
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4
If using a charcoal grill, open vents on bottom of grill, then light charcoal.
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5
Charcoal fire is medium-hot when you can hold your hand 5 inches above rack for 3 to 4 seconds.
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6
If using a gas grill, preheat burners on high, covered, 10 minutes, then reduce heat to moderately high.
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7
While grill heats, stir together coriander, cumin, chili powder, black pepper, and kosher salt in a shallow bowl.
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8
Brush tuna on all sides with 1 tablespoon oil (total), then coat all sides evenly with spice mixture.
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9
Grill tuna on lightly oiled rack, uncovered, turning to brown all sides, until browned on outside but still rare in center, 8 to 10 minutes total.
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10
Let tuna stand 10 minutes.
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11
While tuna stands, heat tortillas on grill, turning over once, until warm, about 1 minute, and keep warm, wrapped in foil.
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12
Toss jicama with cilantro, lime juice, remaining tablespoon oil, and salt and pepper to taste in a small bowl.
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13
Halve, pit, and peel avocado, then cut into 1/2-inch-thick slices.
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14
Put a tortilla on each of 4 plates.
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15
Divide arugula, jicama, and avocado among tortillas, arranging evenly down center of each.
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16
Slice tuna 1/4 inch thick and divide among tacos.
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17
Drizzle chipotle sauce evenly over tuna and roll up tacos to enclose filling.
